The syndrome gets its name from the characteristic cry of affected infants, which is similar to that of a meowing kitten, due to problems with the larynx and nervous system. About 1/3 of children lose the cry by age of 2 years. Other symptoms of cri du chat syndrome may include:

- feeding problems because of difficulty in swallowing and sucking;

- low birth weight and poor growth;

- severe cognitive, speech, and motor delays;

- behavioral problems such as hyperactivity, aggression, outbursts, and repetitive movements;

- unusual facial features which may change over time;

- excessive drooling;

- small head and jaw;

- wide eyes;

- skin tags in front of eyes.

Other common findings include hypotonia, microcephaly, growth retardation, a round face with full cheeks, hypertelorism, epicanthal folds, down-slanting palpebral fissures, strabismus, flat nasal bridge, down-turned mouth, micrognathia, low-set ears, short fingers, single palmar creases, and cardiac defects (e.g., ventricular septal defect [VSD], atrial septal defect [ASD], patent ductus arteriosus [PDA], tetralogy of Fallot). Infertility is not associated with Cri du chat.

It has also been observed that people with the condition have difficulties communicating. While levels of proficiency can range from a few words to short sentences, it is often recommended by medical professionals for the child to undergo some sort of speech therapy/aid with the help of a professional.

Less frequently encountered findings include cleft lip and palate, preauricular tags and fistulas, thymic dysplasia, intestinal malrotation, megacolon, inguinal hernia, dislocated hips, cryptorchidism, hypospadias, rare renal malformations (e.g., horseshoe kidneys, renal ectopia or agenesis, hydronephrosis), clinodactyly of the fifth fingers, talipes equinovarus, pes planus, syndactyly of the second and third fingers and toes, oligosyndactyly, and hyperextensible joints. The syndrome may also include various dermatoglyphics, including transverse flexion creases, distal axial triradius, increased whorls and arches on digits, and a single palmar crease.

Late childhood and adolescence findings include significant intellectual disability, microcephaly, coarsening of facial features, prominent supraorbital ridges, deep-set eyes, hypoplastic nasal bridge, severe malocclusion, and scoliosis.

Affected females reach puberty, develop secondary sex characteristics, and menstruate at the usual time. The genital tract is usually normal in females except for a report of a bicornuate uterus. In males, testes are often small, but spermatogenesis is thought to be normal.

The brain is abnormally smooth, with fewer folds and grooves. The face, especially in children, has distinct characteristics including a short nose with upturned nares, thickened upper lip with a thin vermilion upper border, frontal bossing, small jaw, low-set posteriorily rotated ears, sunken appearance in the middle of the face, widely spaced eyes, and hypertelorism. The forehead is prominent with bitemporal hollowing.

Characteristics that are not visual include mental retardation, pre- and postnatal growth retardation, epilepsy, and reduced lifespan.

Failure to thrive, feeding difficulties, seizures and decreased spontaneous activity are often seen. Death usually occurs in infancy and childhood.

Multiple abnormalities of the brain, kidneys, and gastrointestinal tract (the stomach and intestines) may occur.

The most common characteristics include a distinct craniofacial phenotype (microcephaly, micrognathia, short philtrum, prominent glabella, ocular hypertelorism, dysplastic ears and periauricular tags), growth restriction, intellectual disability, muscle hypotonia, seizures, and congenital heart defects. Less common characteristics include hypospadias, colobomata of the iris, renal anomalies, and deafness. Antibody deficiencies are also common, including common variable immunodeficiency and IgA deficiency. T-cell immunity is normal.

Cri du chat syndrome, also known as chromosome 5p deletion syndrome, 5p− syndrome (pronounced "Five P Minus") or Lejeune’s syndrome, is a rare genetic disorder due to chromosome deletion on chromosome 5. Its name is a French term ("cat-cry" or "call of the cat") referring to the characteristic cat-like cry of affected children. It was first described by Jérôme Lejeune in 1963. The condition affects an estimated 1 in 50,000 live births across all ethnicities and is more common in females by a 4:3 ratio.

The severity of symptoms of idic(15) vary greatly between individuals. Individuals with idic(15) usually have delays in language development and motor skills such as walking or sitting up. Other traits may include low muscle tone (hypotonia), seizures (>50%), short stature, and intellectual disability. Distinctive facial features associated with idic(15), where present, are usually very subtle but may include epicanthal folds (skin folds at the inner corners of one or both eyes), broad forehead, a flattened nasal bridge, button nose, and a high arched palate (roof of the mouth). Many individuals with idic(15) display features of autism, such as problems with communication and social interactions, obsessional interests (often with interactive mechanisms like wheels, doors or switches), unpredictable sleep cycles (and a reduced need for sleep), and repetitive and stereotyped behaviors (e.g., lining up toys, playing with a toy in the same manner over and over again, hand flapping, rocking back and forth). A high pain threshold is often observed. If speech develops, it is often echolalic but some individuals do grasp some language. With a severely affected person there may be an inability to walk or talk.

Recognised symptoms are:

- Only one set of genes on the two chromosomes function (Haploinsufficiency)

- Thrombocytopenia-absent radius (TAR syndrome), in case of a class II-deletion

- Neurological-psychiatric problems: Autism; schizophrenia; epilepsy; learning problems; cognitive disabilities — mild to moderate; developmental delay — mild to moderate (milestones like sitting, standing and walking; come at a later period in childhood); children show an ataxic gait and fall down a lot

- Dysmorphism: Slightly unusual facial appearance; disturbed growth; skeletal malformations; small head (microcephaly); prominent forehead; bulbous nose; deep-set eyes; broad thumbs; broad toes; squint; very flexible joints; clavicular pseudoarthrosis (the collarbone doesn't develop normally) (Class II-deletion); An extra transverse crease of the fifth finger (Class II-deletion)); Problems with the development of the vagina (Müllerian aplasia)

- Eyes: Cataracts

- Heart abnormalities and cardiovascular anomalies (30% of the cases): Anomalous origin of the coronary artery (Class II-deletion)

- Kidneys: Missing kidney or floating kidneys

- Cancer: Neuroblastoma

- Sleep disturbances

It is not clear whether the list of symptoms is complete. Very little information is known about the syndrome. The syndrome can have completely different effects on members of the same family.

A common deletion is between 1.0–1.9Mb. Mefford states that the standard for a deletion is 1.35Mb. The largest deletion seen on a living human is over 5 Mb.

The symptoms associated with this syndrome are variable, but common features include: low birthweight, low muscle tone at birth, poor feeding in infancy (often requiring feeding by tube for a period) and oromotor dyspraxia together with moderate developmental delays and learning disabilities but amiable behaviour. Other clinically important features include epilepsy, heart defects (atrial septal defect, ventricular septal defect) and kidney/urological anomalies. Silvery depigmentation of strands of hair have been noted in several patients. With age there is an apparent coarsening of facial features. 17q21.3 was reported simultaneously in 2006 by three independent groups, with each group reporting several patients, and is now recognised to be one of the more common recurrent microdeletion syndromes. Recently a patient with a small duplication in same segment of DNA has been described. An overview of the clinical features of the syndrome, by reviewing 22 individuals with a 17q21.31 microdeletion, estimated the disorder is present in one in every 16,000 people.

1q21.1 deletion syndrome or 1q21.1 (recurrent) microdeletion is a rare aberration of chromosome 1.

A human cell has one pair of identical chromosomes on chromosome 1. With the 1q21.1 deletion syndrome, one chromosome of the pair is not complete, because a part of the sequence of the chromosome is missing. One chromosome has the normal length and the other is too short.

In 1q21.1, the '1' stands for chromosome 1, the 'q' stands for the long arm of the chromosome and '21.1' stands for the part of the long arm in which the deletion is situated.

The syndrome is a form of the 1q21.1 copy number variations and it is a deletion in the distal area of the 1q21.1 part. The CNV leads to a very variable phenotype and the manifestations in individuals are quite variable. Some people who have the syndrome can function in a normal way, while others have symptoms of mental retardation and various physical anomalies.

1q21.1 microdeletion is a very rare chromosomal condition. Only 46 individuals with this deletion have been reported in medical literature as of August 2011.

Affected individuals present with a broad array of medical and behavioral manifestations (tables 1 and 2). Patients are consistently characterized by global developmental delay, intellectual disability, speech abnormalities, ASD-like behaviors, hypotonia and mild dysmorphic features. Table 1 summarizes the dysmorphic and medical conditions that have been reported in individuals with PMS. Table 2 summarize the psychiatric and neurological associated with PMS. Most of the studies include small samples or relied on parental report or medical record review to collect information, which can account in part for the variability in the presentation of some of the presenting features. Larger prospective studies are needed to further characterize the phenotype.

Table 1: Dysmorphic features and medical comorbid conditions that have been reported in individuals with Phelan McDermid Syndrome.

Table 2: Psychiatric and Neurologic Manifestations associated with Phelan McDermid Syndrome

Potocki–Shaffer syndrome (PSS), also known as DEFECT11 syndrome or chromosome 11p11.2 deletion syndrome, is a rare contiguous gene syndrome that results from the microdeletion of section 11.2 on the short arm of chromosome 11 (11p11.2). The syndrome has its name from Dr. Lorraine (Lori) Potocki and Dr. Lisa Shaffer who discovered the deletion on the 11th chromosome and studied the impacts.

The deletion of this combination of genes results in several distinctive congenital features, occasional defects in the heart, kidneys, and urinary tract. The disorder is associated with an enlarged parietal foramina which can cause openings in the two bones that form the top and sides of the skull. These abnormal openings form extra "soft spots" on the head, in addition to the two that newborns normally have, and unlike the usual newborn soft spots, the enlarged parietal foramina remain open throughout life. Other signs can include multiple mostly noncancerous benign bone tumours called osteochondromas (exostosis), developmental delay, vision disorders and craniofacial abnormalities. It is classified as a rare disease.

The signs and symptoms of Potocki–Shaffer syndrome vary widely. In addition to multiple osteochondromas and enlarged parietal foramina, affected individuals often have intellectual disability and delayed development of speech, motor skills (such as sitting and walking), and social skills. Many people with this condition have distinctive facial features, which can include a wide, short skull (brachycephaly); a prominent forehead; a narrow bridge of the nose; a shortened distance between the nose and upper lip (a short philtrum); and a downturned mouth. Less commonly, Potocki–Shaffer syndrome causes vision problems, additional skeletal abnormalities, and defects in the heart, kidneys, and urinary tract.

17q21.31 microdeletion syndrome (Koolen De Vries syndrome) is a rare genetic disorder caused by a deletion of a segment of chromosome 17 which contains six genes. This deletion syndrome was discovered independently in 2006 by three different research groups.

Miller–Dieker syndrome (abbreviated MDS), Miller–Dieker lissencephaly syndrome (MDLS), and chromosome 17p13.3 deletion syndrome is a micro deletion syndrome characterized by congenital malformations. Congenital malformations are physical defects detectable in an infant at birth which can involve many different parts of the body including the brain, hearts, lungs, liver, bones, or intestinal tract.

MDS is a contiguous gene syndrome - a disorder due to the deletion of multiple gene loci adjacent to one another. The disorder arises from the deletion of part of the small arm of chromosome 17p (which includes both the "LIS1" and "14-3-3 epsilon" genes), leading to partial monosomy. There may be unbalanced translocations (i.e. 17q:17p or 12q:17p), or the presence of a ring chromosome 17.

This syndrome should not be confused with Miller syndrome, an unrelated rare genetic disorder, or Miller Fisher syndrome, a form of Guillain–Barré syndrome.

Wolf–Hirschhorn syndrome (WHS), also known as chromosome deletion Dillan 4p syndrome, Pitt–Rogers–Danks syndrome (PRDS) or Pitt syndrome, was first described in 1961 by Americans Herbert L. Cooper and Kurt Hirschhorn and, thereafter, gained worldwide attention by publications by the German Ulrich Wolf, and Hirschhorn and their co-workers, specifically their articles in the German scientific magazine "Humangenetik". It is a characteristic phenotype resulting from a partial deletion of chromosomal material of the short arm of chromosome 4 (del(4p16.3)).

The clinical phenotype of 3q29 microdeletion syndrome is variable. Clinical features can include mild/moderate mental retardation with mildly dysmorphic facial features (long and narrow face, short philtrum and a high nasal bridge). Of the 6 reported patients, additional features including autism, ataxia, chest-wall deformity and long, tapering fingers were found in at least two patients. A review of 14 children with insterstitial deletions of 3q29, found 11 who had the common recurrent 1.6Mb deletion and displayed mental retardation and microcephaly.

The variability of phenotype is underscored by the report on a 6 and 9/12 year-old male patient with a de novo chromosome 3q29 microdeletion identified by BAC array comparative genomic hybridization assay (aCGH), with accompanying normal 46,XY high-resolution chromosome analysis. The patient has language-based learning disabilities and behavioral features consistent with diagnoses of autism and attention deficit hyperactivity disorder (ADHD) of the inattentive type. He also displays some other features previously associated with chromosome 3q29 microdeletion such as an elongated face, long fingers, and joint laxity. Most notably the patient, per formal IQ testing, was not found to have frank mental retardation as has been previously reported among patients with chromosome 3q29 terminal deletion, but rather the patient has demonstrated an average full-scale IQ result. This report further expands the phenotypic spectrum to include the possibility of normal intelligence as corroborated by formal, longitudinal psycho-educational testing.

The presence of two homologous low copy repeats either side of the deletion break-point suggests that non-allelic homologous recombination is the likely mechanism underlying this syndrome.

8p23.1 duplication syndrome is a rare genetic disorder caused by a duplication of a region from human chromosome 8. This duplication syndrome has an estimated prevalence of 1 in 64,000 births and is the reciprocal of the 8p23.1 deletion syndrome. The 8p23.1 duplication is associated with a variable phenotype including one or more of speech delay, developmental delay, mild dysmorphism, with prominent forehead and arched eyebrows, and congenital heart disease (CHD).

Potocki–Lupski syndrome (PTLS), also known as dup(17)p11.2p11.2 syndrome, trisomy 17p11.2 or duplication 17p11.2 syndrome, is a contiguous gene syndrome involving the microduplication of band 11.2 on the short arm of human chromosome 17 (17p11.2). The duplication was first described as a case study in 1996. In 2000, the first study of the disease was released, and in 2007, enough patients had been gathered to complete a comprehensive study and give it a detailed clinical description. PTLS is named for two researchers involved in the latter phases, Drs. Lorraine Potocki and James R. Lupski of Baylor College of Medicine.

PTLS was the first predicted of a homologous recombination (microdeletion or microduplication) where both reciprocal recombinations result in a contiguous gene syndrome. Its reciprocal disease is Smith–Magenis syndrome (SMS), in which the chromosome portion duplicated in PTLS is deleted altogether.

Potocki–Lupski syndrome is considered a rare disease, predicted to appear in at least 1 in 20,000 humans.

Symptoms of the syndrome include intellectual disability, autism, and other disorders unrelated to the listed symptoms.

22q13 deletion syndrome (spoken as "twenty-two q one three", see Locus (genetics)) is a genetic disorder caused by deletions or rearrangements on the q terminal end (long arm) of chromosome 22. Any abnormal genetic variation in the q13 region that presents with significant manifestations (phenotype) typical of a terminal deletion may be diagnosed as 22q13 deletion syndrome. 22q13 deletion syndrome is often called Phelan-McDermid syndrome (abbreviated PMS). There is disagreement among researchers as to the exact definition of 22q13 deletion syndrome. The Developmental Synaptopathies Consortium defines PMS as being caused by "SHANK3" mutations, a definition that appears to exclude terminal deletions. The requirement to include "SHANK3" in the definition is supported by many, but not by those who first described 22q13 deletion syndrome.

A prototypical terminal deletion of 22q13 can be uncovered by karyotype analysis, but many terminal and interstitial deletions are too small. The availability of DNA microarray technology for revealing multiple genetic problems simultaneously has been the diagnostic tool of choice. The falling cost for whole exome sequencing and, eventually, whole genome sequencing, may replace DNA microarray technology for candidate evaluation. However, fluorescence in situ hybridization (FISH) tests remain valuable for diagnosing cases of mosaicism (mosaic genetics) and chromosomal rearrangements (e.g., ring chromosome, unbalanced chromosomal translocation). Although early researchers sought a monogenic (single gene genetic disorder) explanation, recent studies have not supported that hypothesis (see Etiology, below).

The phenotypic data on 11 patients indicated that cases are not always ascertained for CHD but that CHD was the most common single feature found in 6 out of 11 individuals. Developmental delay and/or learning difficulties were found in 5 out of 11 cases, but one prenatal case was developing normally at 15 months of age (Case 1,). Three other prenatal cases could not yet be reliably assessed. A variable degree of facial dysmorphism was present in 5 out of 11 individuals. Partial toe syndactyly has been found in one mother and son diad and adrenal anomalies in two probands but not in the duplicated mother of one of them. The phenotype is compatible with independent adult life with varying degrees of support.

Duplication of the GATA4 transcription factor () is believed to underlie the congenital heart disease and other genes, common to the duplication and deletion syndromes, can be regarded as candidates for the 8p23.1 duplication syndrome. These include the SOX7 transcription factor () for both CHD and developmental delay and the TNKS gene () for behavioural difficulties. The diaphragmatic hernia found in the 8p23.1 deletion syndrome has not been found in the 8p23.1 duplication syndrome to date.

The duplication may be associated with copy number changes of the adjacent olfactory receptor/defensin repeats (ORDRs) that predispose to the 8p23.1 deletion and duplication syndromes. High total copy numbers of these repeats have been associated with predisposition to psoriasis and low copy number with predisposition to Crohn's disease.

There are many signs and symptoms of Prader–Willi syndrome. The symptoms can range from poor muscle tone during infancy to behavioral problems in early childhood. Some symptoms that are usually found in infants, besides poor muscle tone, would be a lack of eye coordination; some are born with almond-shaped eyes; and due to poor muscle tone the infant may not have a strong sucking reflex. Their cry is weak, and they have difficulty waking up. Another sign of this condition is a thin upper lip.

More aspects seen in a clinical overview include hypotonia and abnormal neurologic function, hypogonadism, developmental and cognitive delays, hyperphagia and obesity, short stature, and behavioral and psychiatric disturbances.

Holm "et al." (1993) describe the following features and signs as pretest indicators of PWS, although not all will be present.

A case was described in 1957 by Michail, Matsoukas and Theodorou. In 1963, Jack Herbert Rubinstein (1925–2006) and Hooshang Taybi (1919–2006) described a larger series of cases.

Typical features of the disorder include:

- Broad thumbs and broad first toes and clinodactyly of the 5th finger

- Mental disability

- Small height, low bone growth, small head

- Cryptorchidism in males

- Unusual facies involving the eyes, nose, and palate

- Anesthesia may be dangerous in these patients: "According to the medical literature, in some cases, individuals with Rubinstein–Taybi syndrome may have complications (e.g., respiratory distress and/or irregular heart beats [cardiac arrythmias]) associated with a certain muscle relaxant (succinylcholine) and certain anesthesia. Any situations requiring the administration of anesthesia or succinylcholine (e.g., surgical procedures) should be closely monitored by skilled professionals (Anesthesiologists)." Primary literature suggests the children may have a higher rate of cardiac physical and conduction abnormalities which may cause unexpected results with cardioactive medications. A further editorial reply in the British Journal of Anaesthesia discusses changes in the face and airway structure making it more difficult to secure the airway under anaesthesia, however, complications appeared in a minority of cases, and routine methods of airway control in the operating room appears to be successful. They recommended close individual evaluation of Rubinstein–Taybi patients for anaesthetic plans.

A 2009 study found that children with RTS were more likely to be overweight and to have a short attention span, motor stereotypies, and poor coordination, and hypothesized that the identified CREBBP gene impaired motor skills learning. Other research has shown a link with long-term memory (LTM) deficit. See also Epigenetics in learning and memory.

3q29 microdeletion syndrome is a rare genetic disorder resulting from the deletion of a segment of chromosome 3. This syndrome was first described in 2005.

Isodicentric 15, also called idic(15), partial tetrasomy 15q, or inverted duplication 15 (inv dup 15), is a chromosome abnormality in which a child is born with extra genetic material from chromosome 15. People with idic(15) are typically born with 47 chromosomes in their body cells, instead of the normal 46. The extra chromosome is made up of a piece of chromosome 15 that has been duplicated end-to-end like a mirror image. It is the presence of this extra genetic material that is thought to account for the symptoms seen in some people with idic(15). Individuals with idic(15) have a total of four copies of this chromosome 15 region instead of the usual two copies (1 copy each on the maternal and paternal chromosomes).

The syndrome is also often referred to by the broader term "Chromosome 15q11.2-q13.1 Duplication Syndrome", shortened to Dup15q syndrome, a name that is supported and actively promoted by the US-based support organization Dup15q Alliance. Dup15q syndrome is a broader disease term, as it includes both idic(15) and interstitial 15q11.2-q13.1, another type of duplication that causes similar clinical traits.

The extra chromosome is occasionally found in the mosaic state, i.e. some of the cells carry the marker chromosome. However, mostly because of the marker's instability and tendency to be lost during cell division (mitosis), some cells are completely normal with 46 chromosomes. Occasionally, cells may have more than one idic(15), resulting in 48 or 49 chromosomes in all or some of their cells. A similar clinical picture albeit to a milder degree could be expected in individuals that have the extra chromosome 15 material as an interstitial duplication (when the extra piece of chromosome 15 is included "within" the long arm of one of the two copies of chromosome 15, rather than as a small extra 'marker' chromosome) - often abbreviated to int dup(15); the individual thus having 46 chromosomes.

Birk-Barel syndrome is a rare genetic disorder associated with the KCNK9 gene. Signs and symptoms include mental retardation, hypotonia, hyperactivity, and syndromic facies.

Due to imprinting, mutations in the maternal copy of KCNK9 cause the conditions, while mutations in the paternal copy do not. As such, this condition can only be inherited from the mother.

Facial features of children with Smith–Magenis syndrome include a broad face, deep-set eyes, large cheeks, and a prominent jaw, as well as a flat nose bridge. The mouth curves downwards and the upper lip curves outwards. These facial features become more noticeable as the individual ages.

Disrupted sleep patterns are characteristic of Smith–Magenis syndrome, typically beginning early in life. Affected people may be very sleepy during the day, but have trouble falling asleep and awaken several times each night, due to an inverted circadian rhythm of melatonin.

People with Smith–Magenis syndrome have engaging personalities, but all also have a lot of behavioral problems. These behavioral problems include frequent temper tantrums, meltdowns and outbursts, aggression, anger, fidgeting, compulsive behavior, anxiety, impulsiveness, and difficulty paying attention. Self-harm, including biting, hitting, head banging, and skin picking, is very common. Repetitive self-hugging is a behavioral trait that may be unique to Smith–Magenis syndrome. People with this condition may also compulsively lick their fingers and flip pages of books and magazines (a behavior known as "lick and flip"), as well as possessing an impressive ability to recall a wide range of small details about people or subject-specific trivia.

Other symptoms can include short stature, abnormal curvature of the spine (scoliosis), reduced sensitivity to pain and temperature, and a hoarse voice. Some people with this disorder have ear abnormalities that lead to hearing loss. Affected individuals may have eye abnormalities that cause nearsightedness (myopia), strabismus, and other problems with vision. Heart and kidney defects also have been reported in people with Smith–Magenis syndrome, though they are less common.

Chromosomal deletion syndromes result from deletion of parts of chromosomes. Depending on the location, size, and whom the deletion is inherited from, there are a few known different variations of chromosome deletions. Chromosomal deletion syndromes typically involve larger deletions that are visible using karyotyping techniques. Smaller deletions result in Microdeletion syndrome, which are detected using fluorescence in situ hybridization (FISH)

Examples of chromosomal deletion syndromes include 5p-Deletion (cri du chat syndrome), 4p-Deletion (Wolf-Hirschhorn syndrome), Prader–Willi syndrome, and Angelman syndrome.