Coffin-Siris syndrome (CSS) is a rare genetic disorder that may be evident at birth (congenital). The disorder may be characterized by abnormalities of the head and facial (craniofacial) area, resulting in a coarse facial appearance. Craniofacial malformations may include an abnormally small head (microcephaly); a wide nose with a low nasal bridge; a wide mouth with thick, prominent lips; thick eyebrows and eyelashes (hypertrichosis); and sparse scalp hair. In addition, affected infants and children typically have short fifth fingers ("pinkies") and toes with underdeveloped (hypoplastic) or absent nails; other malformations of the fingers and toes; and eye abnormalities. Feeding difficulties and frequent respiratory infections during infancy, diminished muscle tone (hypotonia), abnormal looseness (laxity) of the joints, delayed bone age, developmental delays, hearing loss, and intellectual disability may also be present. The specific symptoms and severity can vary among affected individuals. Treatment is directed towards the symptoms that are present in an individual with CSS. Mutations in five different genes, ARID1A, ARID1B, SMARCA4, SMARCB1, and SMARCE1, have been found to cause CSS. Researchers believe the disease can be transmitted genetically as an autosomal dominant trait but most cases appear to be the result of a new mutation.