Polyps are most frequent in the stomach and large intestine, are also found in the small intestine, and are least frequent in the esophagus. A biopsy will reveal them to be hamartomas; the possibility that they progress to cancer is generally considered to be low, although it has been reported multiple times in the past. Chronic diarrhea and protein-losing enteropathy are often observed. Possible collateral features include variable anomalies of ectodermal tissues, such as alopecia, atrophy of the nails, or skin pigmentation

Cronkhite–Canada syndrome is a rare syndrome characterized by multiple polyps of the digestive tract. It is sporadic (i.e. it does not seem to be a hereditary disease), and it is currently considered acquired and idiopathic (i.e. cause remains unknown).

About two-thirds of patients are of Japanese descent and the male to female ratio is 2:1. It was characterized in 1955.

Typically, Hirschsprung's disease is diagnosed shortly after birth, although it may develop well into adulthood, because of the presence of megacolon, or because the baby fails to pass the first stool (meconium) within 48 hours of delivery. Normally, 90% of babies pass their first meconium within 24 hours, and 99% within 48 hours. Other symptoms include green or brown vomit, explosive stools after a doctor inserts a finger into the rectum, swelling of the abdomen, excessive gas, and bloody diarrhea.

Some cases are diagnosed later, into childhood, but usually before age 10. The child may experience fecal retention, constipation, or abdominal distention.

The risks associated with this syndrome include a strong tendency of developing cancer in a number of parts of the body. While the hamartomatous polyps themselves only have a small malignant potential (<3% - OHCM), patients with the syndrome have an increased risk of developing carcinomas of the pancreas, liver, lungs, breast, ovaries, uterus, testicles and other organs.

The average age of first diagnosis is 23, but the lesions can be identified at birth by an astute pediatrician or family physician. Prior to puberty, the mucocutaneous lesions can be found on the palms and soles. Often the first presentation is a bowel obstruction from an intussusception which is a common cause of mortality; an intussusception is a telescoping of one loop of bowel into another segment.

Peutz–Jeghers syndrome (often abbreviated PJS) is an autosomal dominant genetic disorder characterized by the development of benign hamartomatous polyps in the gastrointestinal tract and hyperpigmented macules on the lips and oral mucosa (melanosis). This syndrome can be classed as one of various hereditary intestinal polyposis syndromes and one of various hamartomatous polyposis syndromes. It has an incidence of approximately 1 in 25,000 to 300,000 births.

Hirschsprung's disease (HD or HSCR) is a birth defect in which nerves are missing from parts of the intestine. The most prominent symptom is constipation. Other symptoms may include vomiting, abdominal pain, diarrhea, and slow growth. Symptoms usually become apparent in the first two months of life. Complications may include enterocolitis, megacolon, bowel obstruction, and intestinal perforation.

The disorder may occur by itself or in association with other genetic disorders such as Down syndrome or Waardenburg syndrome. About half of isolated cases are linked to a specific genetic mutation and about 20% occur within families. Some of these occur in an autosomal dominant manner. The cause of the remaining cases is unclear. If otherwise normal parents have one child with the condition, the next child has a 4% risk of being affected. The condition is divided into two main types short-segment and long-segment depending on how much of the bowel is affected. Rarely the small bowel maybe affected as well. Diagnosis is based on symptoms and confirmed by biopsy.

Treatment is generally by surgery to remove the affected section of bowel. The surgical procedure most often carried out is known as a "pull through". Occasionally an intestinal transplantation may be recommended. Hirschsprung's disease occurs in about one in 5,000 of newborns. Males are more often affected than females. The condition is believed to have first been described in 1691 by Frederik Ruysch.

Hereditary diffuse gastric cancer is inherited as an autosomal dominant mutation of the E-cadherin gene (CDH1), which is located on chromosome 16q22.1.

Surgical removal of the stomach (gastrectomy) is typically recommended after for people after 20 years of age, and before 40 years of age.

The differential diagnosis is quite extensive and includes

- Buschke–Fischer–Brauer disease

- Curth–Macklin ichthyosis

- Gamborg Nielsen syndrome

- Greither disease

- Haber syndrome

- Hereditary punctate palmoplantar keratoderma

- Jadassohn–Lewandowsky syndrome

- Keratosis follicularis spinulosa decalvans

- Keratosis linearis with ichthyosis congenital and sclerosing keratoderma syndrome

- Meleda disease

- Mucosa hyperkeratosis syndrome

- Naegeli–Franceschetti–Jadassohn syndrome

- Naxos disease

- Olmsted syndrome

- Palmoplantar keratoderma and leukokeratosis anogenitalis

- Pandysautonomia

- Papillomatosis of Gougerot and Carteaud

- Papillon–Lefèvre syndrome

- Punctate porokeratotic keratoderma

- Richner–Hanhart syndrome

- Schöpf–Schulz–Passarge syndrome

- Unna Thost disease

- Vohwinkel syndrome

- Wong's dermatomyositis

Howel–Evans syndrome is an extremely rare condition involving thickening of the skin in the palms of the hands and the soles of the feet (hyperkeratosis). This familial disease is associated with a high lifetime risk of esophageal cancer. For this reason, it is sometimes known as tylosis with oesophageal cancer (TOC).

The condition is inherited in an autosomal dominant manner, and it has been linked to a mutation in the "RHBDF2" gene. It was first described in 1958.

Bannayan–Riley–Ruvalcaba syndrome is associated with enlarged head and benign mesodermal hamartomas (multiple hemangiomas, and intestinal polyps). Dysmorphy as well as delayed neuropsychomotor development can also be present. The head enlargement does not cause widening of the ventricles or raised intracranial pressure; these individuals have a higher risk of developing tumors, as the gene involved in BRRs is phosphatase and tensin homologue.

Some individuals have thyroid issues consistent with multinodular goiter, thyroid adenoma, differentiated non-medullary thyroid cancer,

most lesions are slowly growing. Visceral as well as intracranial involvement may occur in some cases, and can cause bleeding and symptomatic mechanical compression

The symptoms of blue diaper syndrome could include irritability, constipation, poor appetite, vomiting, and poor growth. Some children experience frequent fevers and intestinal infections.

Hypercalcemia could be a potential issue in affected children. Some children with blue diaper syndrome have eye or vision issues, particularly underdeveloped portions of the eye, including the cornea and optic disc.

Hereditary nonpolyposis colorectal cancer (HNPCC) or Lynch syndrome is an autosomal dominant genetic condition that has a high risk of colon cancer as well as other cancers including endometrial cancer (second most common), ovary, stomach, small intestine, hepatobiliary tract, upper urinary tract, brain, and skin. The increased risk for these cancers is due to inherited mutations that impair DNA mismatch repair. It is a type of cancer syndrome.

Often AGID is a symptom of other problems, including colon cancer, lupus, lung, breast, or ovarian carcinoma or thymoma. or other diseases. Irritable bowel syndrome (IBS) is the most recognized form of AGID.

From early adolescence, patients with this condition gradually (and much of the time 'silently') develop hundreds to thousands of colorectal polyps (and sometimes polyps elsewhere)—small abnormalities at the surface of the intestinal tract, especially in the large intestine including the colon or rectum. These may bleed, leading to blood in the stool. If the blood is not visible, it is still possible for the patient to develop anemia due to gradually developing iron deficiency. If malignancy develops, this may present with weight loss, altered bowel habit, or even metastasis to the liver or elsewhere. FAP can also develop 'silently' in some individuals, giving few or no signs until it has developed into advanced colorectal cancer.

Because familial polyposis develops very gradually over years, and can also manifest in an 'attenuated' form even more gradually, polyps resulting from FAP can lead to cancer developing at any point from adolescence to old age.

Depending on the nature of the defect in the APC gene, and whether it is the full or attenuated form, familial polyposis may manifest as polyps in colon or in the duodenal tract, or in any combination of these. Therefore, an absence of polyps in, for example, the rectum, may not of itself be sufficient to confirm absence of polyps. It may be necessary to consider and visually examine other possible parts of the intestinal tract. Colonoscopy is preferred over sigmoidoscopy for this, as it provides better observation of the common right-side location of polyps.

The genetic determinant in familial polyposis may also predispose carriers to other malignancies, e.g., of the duodenum and stomach (particularly ampullary adenocarcinoma). Other signs that may point to FAP are pigmented lesions of the retina ("CHRPE—congenital hypertrophy of the retinal pigment epithelium"), jaw cysts, sebaceous cysts, and osteomata (benign bone tumors). The combination of polyposis, osteomas, fibromas and sebaceous cysts is termed "Gardner's syndrome" (with or without abnormal scarring).

The Amsterdam clinical criteria identifies candidates for genetic testing, and genetic testing can make a diagnosis of Lynch syndrome. Genetic testing is commercially available and consists of a blood test.

Bannayan–Riley–Ruvalcaba syndrome (BRRS) is a rare overgrowth syndrome and hamartomatous disorder with occurrence of multiple subcutaneous lipomas, macrocephaly and hemangiomas. The disease is inherited in an autosomal dominant manner.

The disease belongs to a family of hamartomatous polyposis syndromes, which also includes Peutz–Jeghers syndrome, juvenile polyposis and Cowden syndrome. Mutation of the PTEN gene underlies this syndrome, as well as Cowden syndrome, Proteus syndrome, and Proteus-like syndrome, these four syndromes are referred to as PTEN Hamartoma-Tumor Syndromes.

Gardner syndrome, also known as Gardner's syndrome or familial colorectal polyposis, is an autosomal dominant form of polyposis characterized by the presence of multiple polyps in the colon together with tumors outside the colon. The extracolonic tumors may include osteomas of the skull, thyroid cancer, epidermoid cysts, fibromas, as well as the occurrence of desmoid tumors in approximately 15% of affected individuals.

Desmoid tumors are fibrous tumors which usually occur in the tissue covering the intestines and may be provoked by surgery to remove the colon. The countless polyps in the colon predispose to the development of colon cancer; if the colon is not removed, the chance of colon cancer is considered to be very significant. Polyps may also grow in the stomach, duodenum, spleen, kidneys, liver, mesentery and small bowel. In a small number of cases, polyps have also appeared in the cerebellum. Cancers related to Gardner syndrome commonly appear in the thyroid, liver and kidneys. The number of polyps increases with age, and hundreds to thousands of polyps can develop in the colon.

The syndrome was first described in 1951. There is no cure at this time, and in its more advanced forms, it is considered a terminal diagnosis with a life expectancy of 35–45 years; treatments are surgery and palliative care, although some chemotherapy has been tried with limited success.

Blue diaper syndrome is a rare, autosomal recessive metabolic disorder characterized in infants by bluish urine-stained diapers. It is also known as Drummond's syndrome, and hypercalcemia.

It is caused by a defect in tryptophan absorption. Bacterial degradation of unabsorbed tryptophan in the intestine leads to excessive indole production and thus to indicanuria which, on oxidation to indigo blue, causes a peculiar bluish discoloration of the diaper (indoluria). Symptoms typically include digestive disturbances, fever and visual problems. Some may also develop disease due to the incomplete breakdown of tryptophan.

It was characterized in 1964, and is associated with the X linked sex gene.

Since this syndrome is X linked, the chance for a child to receive normal genes from both parents and be genetically normal for that particular trait is 25%. If an individual receives one normal gene and one gene for the disease, the person will be a carrier for the disease, but usually will not show symptoms. Carrier females usually do not display symptoms of the disorder because it is usually the X chromosome with the abnormal gene that is “turned off”. Parents can undergo genetic testing to see if their child will get this syndrome, but most do not find out until they see the symptoms mentioned below.

AGID is diagnosed with a complete medical history, exam of patients motility and with special blood tests looking for autoantibodies consistent with neurologic autoimmunity. Blood tests included evaluations of immunofluorescence (neuronal nuclear and cytoplasmic antibodies), radioimmunoprecipitation assays (neuronal and muscle plasma membrane cation channel antibodies), and enzyme-linked immunosorbent assay (muscle striational antibodies). A finding, along with medical history, of ganglionic neuronal acetylcholine receptor and N-type voltage-gated calcium channel autoantibodies in the blood stream would result in a medically acceptable diagnosis of AGID.

AMS has been described by multiple authors and institutions, and various definitions have been adopted. According to Newton et al., a scoring system allotting one point per feature establishes AMS with scores greater than or equal to 3. The features include: 1) two or more clinically atypical nevi, 2) more than 100 nevi in patients between 20 and 50 years of age, 3) more than 50 nevi in patients under 20 years of age or more than 50 years of age, 4) more than one nevus in buttocks or instep, 5) nevi on the anterior scalp, 6) one or more pigmented lesions in the iris.

The Classical (1990) definition uses the following criteria: 1) 100 or more melanocytic nevi, 2) one or more melanocytic nevi greater than or equal to 8mm in its largest diameter, and 3) one or more clinically atypical melanocytic nevi.

The National Institutes of Health (NIH) Consensus 1992 definition, which is still controversial, requires a family history of melanoma, in addition to a large number of melanocytic nevi (often greater than 50) and melanocytic nevi that present certain histological features.

The symptoms and prognosis of tetrasomy 9p are highly variable. The severity of the symptoms is largely determined by the size of the isochromosome, the specific regions of chromosome 9p that are duplicated, as well as the number and type of tissues that are affected in the mosaic form.

Most patients exhibit some degree of intellectual disability, abnormal skeletal and muscular development, and abnormal facial structures. Cognitive symptoms range from slight learning disabilities to severe deficits in intellectual functioning. Due to abnormal development of the muscles, individuals often experience limited or delayed mobility. Atypical facial features are characteristic of the syndrome, including widely spaced eyes, a large nose, and unusually positioned ears. Additionally, patients often have extra skin around the neck and widely spaced nipples. A wide range of renal, digestive, cardiac, respiratory, and nervous system abnormalities have been observed.

Though rare, a few cases of phenotypically normal individuals with tetrasomy 9p have been documented.

In 1820 Norris reported the first case of what is now recognized as FAMMM (12). He described a 59-year-old man with melanoma, a high total body mole count, and family history of the same.

The skin is normal at birth. Between 3 and 6 months of age, the affected carrier develops poikiloderma on the cheeks. This characteristic "rash" that all RTS carriers have can develop on the arms, legs and buttocks. "Poikiloderma consists of areas of increased and decreased pigmentation, prominent blood vessels, and thinning of the skin."

This syndrome is characterized by overgrowth and advanced bone age. Affected individuals are dysmorphic, with macrodolichocephaly, downslanting palpebral fissures and a pointed chin. The facial appearance is most notable in early childhood. Affected infants and children tend to grow quickly; they are significantly taller than their siblings and peers, and have an unusually large skull and large head. Adult height is usually in the normal range, although Broc Brown has the condition and was named the world's tallest teenager. As of late 2016, he was 7'8" and still growing.

Individuals with Sotos syndrome often have intellectual impairment, and most also have behavioral problems. Frequent behavioral impairments include attention deficit hyperactivity disorder (ADHD), phobias, obsessive compulsive disorder, tantrums, and impulsive behaviors (impulse control disorder). Problems with speech and language are also common. Affected individuals may often have stuttering, difficulty with sound production, or a monotone voice. Additionally, weak muscle tone (hypotonia) may delay other aspects of early development, particularly motor skills such as sitting and crawling.

Other signs include scoliosis, seizures, heart or kidney defects, hearing loss, and problems with vision. Some infants with this disorder experience jaundice and poor feeding. A small number of patients with Sotos syndrome have developed cancer, most often in childhood, but no single form of cancer has been associated with this condition. It remains uncertain whether Sotos syndrome increases the risk of specific types of cancer. If persons with this disorder have any increased cancer risk, their risk is only slightly greater than that of the general population.