Even in syndromes with no known etiology, the presence of the associated symptoms with a statistically improbable correlation, normally leads the researchers to hypothesize that there exists an unknown underlying cause for all the described symptoms.

Roberts syndrome is an extremely rare condition that only affects about 150 reported individuals. Although there have been only about 150 reported cases, the affected group is quite diverse and spread worldwide. Parental consanguinity (parents are closely related) is common with this genetic disorder. The frequency of Roberts syndrome carriers is unknown.

Males with Down syndrome usually do not father children, while females have lower rates of fertility relative to those who are unaffected. Fertility is estimated to be present in 30–50% of females. Menopause typically occurs at an earlier age. The poor fertility in males is thought to be due to problems with sperm development; however, it may also be related to not being sexually active. As of 2006, three instances of males with Down syndrome fathering children and 26 cases of females having children have been reported. Without assisted reproductive technologies, around half of the children of someone with Down syndrome will also have the syndrome.

More than 80% of children with Patau syndrome die within the first year of life. Children with the mosaic variation are usually affected to a lesser extent. In a retrospective Canadian study of 174 children with trisomy 13, median survival time was 12.5 days. One and ten year survival was 19.8% and 12.9% respectively.

Between 5 and 15% of children with Down syndrome in Sweden attend regular school. Some graduate from high school; however, most do not. Of those with intellectual disability in the United States who attended high school about 40% graduated. Many learn to read and write and some are able to do paid work. In adulthood about 20% in the United States do paid work in some capacity. In Sweden, however, less than 1% have regular jobs. Many are able to live semi-independently, but they often require help with financial, medical, and legal matters. Those with mosaic Down syndrome usually have better outcomes.

Individuals with Down syndrome have a higher risk of early death than the general population. This is most often from heart problems or infections. Following improved medical care, particularly for heart and gastrointestinal problems, the life expectancy has increased. This increase has been from 12 years in 1912, to 25 years in the 1980s, to 50 to 60 years in the developed world in the 2000s. Currently between 4 and 12% die in the first year of life. The probability of long-term survival is partly determined by the presence of heart problems. In those with congenital heart problems 60% survive to 10 years and 50% survive to 30 years of age. In those without heart problems 85% survive to 10 years and 80% survive to 30 years of age. About 10% live to 70 years of age. The National Down Syndrome Society have developed information regarding the positive aspects of life with Down syndrome.

Nevo Syndrome is considered to be a rare disorder. Since its first appearance in 1974, only a handful of cases have been reported. Studies have shown showing similarities between Nevo Syndrome with Ehlers-Danlos syndrome as well as Sotos syndrome. There is an astounding overlap of phenotypic manifestations between Nevo Syndrome and the more frequent Sotos syndrome, which are both caused by the NSD1 deletion. Sotos syndrome is an autosomal dominant condition associated with learning disabilities, a distinctive facial appearance, and overgrowth. Studies have shown an overwhelming occurrence (half of those involved in the study) of Nevo syndrome in those individuals of Middle-Eastern descent.

As the syndrome is due to a chromosomal non-disjunction event, the recurrence risk is not high compared to the general population. There has been no evidence found that indicates non-disjunction occurs more often in a particular family.

Pashayan syndrome also known as Pashayan–Prozansky Syndrome, and blepharo-naso-facial syndrome is a rare syndrome. Facial abnormalities characterise this syndrome as well as malformation of extremities. Specific characteristics would be a bulky, flattened nose, where the face has a mask like appearance and the ears are also malformed.

A subset of Pashayan syndrome has also been described, known as "cerebrofacioarticular syndrome", "Van Maldergem syndrome'" or "Van Maldergem–Wetzburger–Verloes syndrome". Similar symptoms are noted in these cases as in Pashayan syndrome.

Documented cases of Reye syndrome in adults are rare. The recovery of adults with the syndrome is generally complete, with liver and brain function returning to normal within two weeks of onset. In children, however, mild to severe permanent brain damage is possible, especially in infants. Over thirty percent of the cases reported in the United States from 1981 through 1997 resulted in fatality.

Reye syndrome occurs almost exclusively in children. While a few adult cases have been reported over the years, these cases do not typically show permanent neural or liver damage. Unlike in the UK, the surveillance for Reye syndrome in the US is focused on patients under 18 years of age.

In 1980, after the CDC began cautioning physicians and parents about the association between Reye syndrome and the use of salicylates in children with chickenpox or virus-like illnesses, the incidence of Reye syndrome in the United States began to decline. However, the decline began prior to the FDA's issue of warning labels on aspirin in 1986. In the United States between 1980 and 1997, the number of reported cases of Reye syndrome decreased from 555 cases in 1980 to about 2 cases per year since 1994. During this time period 93% of reported cases for which racial data were available occurred in whites and the median age was six years. In 93% of cases a viral illness had occurred in the preceding three-week period. For the period 1991-1994, the annual rate of hospitalizations due to Reye syndrome in the US was estimated to be between 0.2 and 1.1 per million population less than 18 years of age.

During the 1980s, a case-control study carried out in the United Kingdom also demonstrated an association between Reye syndrome and aspirin exposure. In June 1986, the United Kingdom Committee on Safety of Medicines issued warnings against the use of aspirin in children under 12 years of age and warning labels on aspirin-containing medications were introduced. UK surveillance for Reye syndrome documented a decline in the incidence of the illness after 1986. The reported incidence rate of Reye syndrome decreased from a high of 0.63 per 100,000 population less than 12 years of age in 1983/84 to 0.11 in 1990/91.

From November 1995 to November 1996 in France, a national survey of pediatric departments for children under 15 years of age with unexplained encephalopathy and a threefold (or greater) increase in serum aminotransferase and/or ammonia led to the identification of nine definite cases of Reye syndrome (0.79 cases per million children). Eight of the nine children with Reye syndrome were found to have been exposed to aspirin. In part because of this survey result, the French Medicines Agency reinforced the international attention to the relationship between aspirin and Reye syndrome by issuing its own public and professional warnings about this relationship.

The prognosis for patients diagnosed with Timothy syndrome is very poor. Of 17 children analyzed in one study, 10 died at an average age of 2.5 years. Of those that did survive, 3 were diagnosed with autism, one with an autism spectrum disorder, and the last had severe delays in language development. One patient with atypical Timothy syndrome was largely normal with the exception of heart arrhythmia. Likewise, the mother of two Timothy syndrome patients also carried the mutation but lacked any obvious phenotype. In both of these cases, however, the lack of severity of the disorder was due to mosaicism.

Unless one of the parents is a carrier of a translocation, the chances of a couple having another trisomy 13 affected child is less than 1% (less than that of Down syndrome). The most common characteristics of this syndrome are problems such as late development, mental disability, multiple malformations, cardiopathy, and kidney abnormalities. The most common physical signs for Patau Syndrome are the decreasing of muscle tone, small hands, small ears, small head and mouth, as well as wide and short hands with short fingers. Physical development for children affected by Patau Syndrome occurs more slowly than children without Patau syndrome. However, children affected by Patau Syndrome should still undergo regular physical activity, even though muscle development may occur more slowly.

In medicine a broad definition of syndrome is used, which describes a collection of symptoms and findings without necessarily tying them to a single identifiable pathogenesis. The more specific definition employed in medical genetics describes a subset of all medical syndromes.

Nevo Syndrome is an autosomal recessive disorder. Most times in which a child is afflicted with Nevo Syndrome, both their parents are of average height and weight. It is only until after birth when the characteristic physical traits associated with disease are manifested, and the disorder is actually diagnosed. One study showed that despite the increased growth rates, the patient was completely healthy up until age 6, when he was admitted into the hospital. Nevo syndrome is usually associated with early childhood fatality. Children with Nevo Syndrome have a high occurrence of death due to cardiac arrest because their developing hearts cannot keep up with their overgrown body.

The prognosis varies widely from case to case, depending on the severity of the symptoms. However, almost all people reported with Aicardi syndrome to date have experienced developmental delay of a significant degree, typically resulting in mild to moderate to profound intellectual disability. The age range of the individuals reported with Aicardi syndrome is from birth to the mid 40s.

There is no cure for this syndrome.

Turner syndrome occurs in between one in 2000 and one in 5000 females at birth.

Approximately 99 percent of fetuses with Turner syndrome spontaneously terminate during the first trimester. Turner syndrome accounts for about 10 percent of the total number of spontaneous abortions in the United States.

In the United States, sarcoidosis has a prevalence of approximately 10 cases per 100,000 whites and 36 cases per 100,000 blacks. Heerfordt syndrome is present in 4.1–5.6% of those with sarcoidosis.

Williams syndrome is a microdeletion syndrome caused by the spontaneous deletion of genetic material from the region q11.23 of one member of the pair of chromosome 7, so that the person is hemizygous for those genes. The deleted region includes more than 25 genes, and researchers believe that being hemizygous for these genes probably contributes to the characteristic features of this syndrome. "CLIP2", "ELN", "GTF2I", "GTF2IRD1", and "LIMK1" are among the genes that are typically deleted from one chromosome in people with Williams syndrome. Researchers have found this hemizygosity for the "ELN" gene, which codes for the protein elastin, is associated with the connective-tissue abnormalities and cardiovascular disease (specifically supravalvular aortic stenosis and supravalvular pulmonary stenosis) found in many people with this syndrome. The insufficient supply of elastin may also be the cause of full cheeks, harsh or hoarse voice, hernias and bladder diverticula often found in those with Williams syndrome. Studies suggest that hemizygosity in "LIMK1", "GTF2I", "GTF2IRD1", and perhaps other genes may help explain the characteristic difficulties with visual–spatial tasks. Additionally, there is evidence that the hemizygosity in several of these genes, including "CLIP2", may contribute to the unique behavioral characteristics, learning disabilities, and other cognitive difficulties seen in Williams syndrome.

The incidence of Fraser syndrome is 0.043 per 10,000 live born infants and 1.1 in 10,000 stillbirths, making it a rare syndrome.

There is no specific treatment or cure for individuals affected with this type of syndrome, though some of the abnormal physical features may be surgically correctable.

3C syndrome is very rare, occurring in less than 1 birth per million. Because of consanguinity due to a founder effect, it is much more common in a remote First Nations village in Manitoba, where 1 in 9 people carries the recessive gene.

Williams syndrome has historically been estimated to occur in roughly 1 in every 20,000 live births. However, more recent epidemiological studies have placed the occurrence rate at closer to 1 in every 7,500 live births, a significantly larger prevalence. As an increasing body of evidence suggests that Williams syndrome is more common than originally noted (approximately 6% of all genetic cases of developmental disability), researchers have begun to theorize past under-diagnosis of the syndrome. One theorized reason for the increase in epidemiological estimates is that there exists a substantial minority of individuals with the genetic markers of Williams syndrome who lack the characteristic facial features or the diminished IQ considered to be diagnostic of the syndrome, who often are not immediately recognized as people with the syndrome.

Marshall–Smith syndrome is not to be confused with:

- Marshall syndrome (aka.Periodic fever, aphthous stomatitis, pharyngitis and adenitis (PFAPA syndrome, see also: Periodic fever syndrome)

- Sotos (like) syndrome

- Weaver-Smith syndrome (WSS)

Some people may have some mental slowness, but children with this condition often have good social skills. Some males may have problems with fertility.

Spanish researchers reported the development of a Costello mouse, with the G12V mutation, in early 2008. Although the G12V mutation is rare among children with Costello syndrome, and the G12V mouse does not appear to develop tumors as expected, information about the mouse model's heart may be transferrable to humans.

Italian and Japanese researchers published their development of a Costello zebrafish in late 2008, also with the G12V mutation. The advent of animal models may accelerate identification of treatment options.