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Outside of the nucleus, the PHF8 protein cannot interact with chromatin to regulate gene expression. While the exact disease mechanism is unknown, it is likely that a lack of PHF8 protein in the nucleus of brain cells before birth prevents chromatin remodeling, altering the normal expression of genes involved in intellectual function and formation of structures along the midline of the skull. This altered gene expression leads to intellectual disability, cleft lip and palate, and the other features of X-linked intellectual disability, Siderius type.</p><p class=\"section-source\"><a href=\"https://medlineplus.gov/genetics/condition/x-linked-intellectual-disability-siderius-type/#causes\">MedlinePlus Genetics: Causes</a></p>","key":"forms","title":"What causes X-linked intellectual disability, Siderius type?"},{"bodyHtml":"<p>Individuals with X-linked intellectual disability, Siderius type frequently also have an opening in the lip (cleft lip) with an opening in the roof of the mouth (cleft palate). A cleft can occur on one or both sides of the upper lip.</p><p>Some boys and men with this condition have distinctive facial features, including a long face, a sloping forehead, a broad nasal bridge, a prominent bone in the lower forehead (supraorbital ridge), and outside corners of the eyes that point upward (upslanting palpebral fissures). Affected individuals may also have low-set ears and large hands.</p><p class=\"section-source\">Source: <a href=\"https://medlineplus.gov/genetics/condition/x-linked-intellectual-disability-siderius-type\">MedlinePlus Genetics</a></p>","key":"overview","title":"Condition overview"}],"sharedSources":[],"slug":"x-linked-intellectual-disability-siderius-type","sourceDocuments":[{"name":"X-linked intellectual disability, Siderius type","reviewed":"2015-06","sections":{"causes":{"fetchedAt":"2026-10-20T20:25:58.327904+00:00","label":"Causes","pageSha256":"7baa3dc0b2bb4a6adb5c56c17c491886fa891e83995b5ac8829ca8cce7ad35cc","sourceUrl":"https://medlineplus.gov/genetics/condition/x-linked-intellectual-disability-siderius-type/#causes","status":"available"}},"source_release":"2026-10-01","url":"https://medlineplus.gov/genetics/condition/x-linked-intellectual-disability-siderius-type"}],"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":"X-linked intellectual disability, Siderius type"},"schemaVersion":1}