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pigmentation","onset_id":null,"reference":"ORPHA:626","source_release":"2026-06-23"},"sourceGroups":["orphanet"]},{"frequencyKind":"reported","key":"fab36da522f97760bf37e0afb5ce081ffffd487df053e12a8c636e89d46b77e7","raw":{"aspect":"P","body_system":"head_and_neck","disease_id":"OMIM:137550","disease_name":"Melanocytic nevus syndrome, congenital","evidence":"TAS","frequency_raw":null,"hpo_id":"HP:0000418","is_excluded":false,"label":"Narrow nasal ridge","onset_id":null,"reference":"OMIM:137550","source_release":"2026-06-23"},"sourceGroups":["omim"]}],"referenceLabels":{},"references":{"causeHtml":"<ul><li><a href=\"https://medlineplus.gov/genetics/condition/giant-congenital-melanocytic-nevus\">Giant congenital melanocytic nevus</a></li></ul>","identityNote":"MONDO cross-references identify native condition entries. Ancestor membership does not transfer phenotype values between groups.","importantHtml":""},"sections":[{"bodyHtml":"<p>NRAS gene mutations cause most cases of giant congenital melanocytic nevus. Rarely, mutations in the BRAF gene are responsible for this condition.</p><p>The proteins produced from these genes are involved in a process known as signal transduction by which signals are relayed from outside the cell to the cell&#x27;s nucleus. Signals relayed by the N-Ras and BRAF proteins instruct the cell to grow and divide (proliferate) or to mature and take on specialized functions (differentiate). To transmit signals, these proteins must be turned on; when the proteins are turned off, they do not relay signals to the cell&#x27;s nucleus.</p><p>The NRAS or BRAF gene mutations responsible for giant congenital melanocytic nevus are somatic, meaning that they are acquired during a person&#x27;s lifetime and are present only in certain cells. These mutations occur early in embryonic development during the growth and division (proliferation) of cells that develop into melanocytes. Somatic NRAS or BRAF gene mutations cause the altered protein in affected cells to be constantly turned on (constitutively active) and relaying signals. The overactive protein may contribute to the development of giant congenital melanocytic nevus by allowing cells that develop into melanocytes to grow and divide uncontrollably, starting before birth.</p><p class=\"section-source\"><a href=\"https://medlineplus.gov/genetics/condition/giant-congenital-melanocytic-nevus/#causes\">MedlinePlus Genetics: Causes</a></p>","key":"forms","title":"What causes Giant congenital melanocytic nevus?"},{"bodyHtml":"<p>People with giant congenital melanocytic nevus may have more than one nevus (plural: nevi). The other nevi are often smaller than the giant nevus. Affected individuals may have one or two additional nevi or multiple small nevi that are scattered over the skin; these are known as satellite or disseminated nevi.</p><p>Affected individuals may feel anxiety or emotional stress due to the impact the nevus may have on their appearance and their health. Children with giant congenital melanocytic nevus can develop emotional or behavior problems.</p><p>Some people with giant congenital melanocytic nevus develop a condition called neurocutaneous melanosis, which is the presence of pigment-producing skin cells (melanocytes) in the tissue that covers the brain and spinal cord. These melanocytes may be spread out or grouped together in clusters. Their growth can cause increased pressure in the brain, leading to headache, vomiting, irritability, seizures, and movement problems. Tumors in the brain may also develop.</p><p>Individuals with giant congenital melanocytic nevus have an increased risk of developing an aggressive form of skin cancer called melanoma, which arises from melanocytes. Estimates vary, but it is generally thought that people with giant congenital melanocytic nevus have a 5 to 10 percent lifetime risk of developing melanoma. Melanoma commonly begins in the nevus, but it can develop when melanocytes that invade other tissues, such as those in the brain and spinal cord, become cancerous. When melanoma occurs in people with giant congenital melanocytic nevus, the survival rate is low.</p><p>Other types of tumors can also develop in individuals with giant congenital melanocytic nevus, including soft tissue tumors (sarcomas), fatty tumors (lipomas), and tumors of the nerve cells (schwannomas).</p><p class=\"section-source\">Source: <a href=\"https://medlineplus.gov/genetics/condition/giant-congenital-melanocytic-nevus\">MedlinePlus Genetics</a></p>","key":"overview","title":"Condition overview"}],"sharedSources":[],"slug":"giant-congenital-melanocytic-nevus","sourceDocuments":[{"name":"Giant congenital melanocytic nevus","reviewed":"2014-12","sections":{"causes":{"fetchedAt":"2026-10-20T20:21:45.413192+00:00","label":"Causes","pageSha256":"d2be1c2f3faf59548e01e59a2a65f91c76da129812605f77e0adbd77c058aafc","sourceUrl":"https://medlineplus.gov/genetics/condition/giant-congenital-melanocytic-nevus/#causes","status":"available"}},"source_release":"2026-10-01","url":"https://medlineplus.gov/genetics/condition/giant-congenital-melanocytic-nevus"}],"sourceNote":"MedlinePlus descriptions and Causes; original HPO annotations.","takeawayHtml":"Cause explanations are from MedlinePlus Genetics. Feature values below retain their original sources and patient groups.","title":"Giant congenital melanocytic nevus"},"schemaVersion":1}