C syndrome is a rare multiple congenital anomaly/intellectual disability syndrome characterized by trigonocephaly and metopic suture synostosis, dysmorphic facial features, short neck, skeletal anomalies, and variable intellectual disability. The disease is Monarch Disease Ontology id MONDO_0008893 (C syndrome). Also known as: OTCS, Opitz C trigonocephaly, Opitz trigonocephaly C syndrome, Opitz trigonocephaly syndrome, trigonocephaly C syndrome.