C syndrome is a rare congenital disorder characterised by trigonocephaly, severe intellectual deficit, and various facial, skeletal, and cardiac abnormalities. The facial abnormalities consist of upslanting palpebral fissures, thick epicanthic folds, frontal midline capillary hemangioma, strabismus, depressed nasal bridge with a short/thick nasal septum and columella, flat philtrum, thin vermilion borders, micrognathia, highly arched palate, buccal-gingival frenula, and multiple ear abnormalities. C syndrome is diagnosed using brain MRI, echocardiography or ultrasonography. The principle differential diagnosis for C syndrome is Smith-Lemli-Opitz Syndrome, which shares many common/overlapping features with C syndrome.

C syndrome is inherited in an autosomal dominant or autosomal recessive manner. A few cases are known to be associated with mutations in CD96 gene. However, most of the reported cases are sporadic.