Individuals with trigonocephaly have a keel-shaped forehead with wide biparietal diameter, resulting in a triangular shape of the head. Trigonocephaly results from premature closure of the metopic sutures and usually occurs sporadically (summary by Frydman et al., 1984). Genetic Heterogeneity of Isolated Trigonocephaly Also see trigonocephaly-2 (TRIGNO2; 614485), caused by mutation in the FREM1 gene (608944) on chromosome 9p22.

Phosphoribosylaminoimidazole carboxylase deficiency (PAICSD) is an autosomal recessive disorder characterized by multiple congenital anomalies and early neonatal death (Pelet et al., 2019).

Faundes-Banka syndrome (FABAS) is an autosomal dominant disorder characterized by variable combinations of developmental delay and microcephaly, as well as micrognathia and other dysmorphic features (Faundes et al., 2021).