Mutations in PGAP3 result in hyperphosphatasia with the mental retardation-4 disorder (HPMRS4, OMIM # 615716), a rare autosomal recessive neurologic disorder characterized by structural brain anomalies, severely delayed psychomotor development, mental retardation, hypotonia, seizures, lack of speech acquisition, and dysmorphic facial features. The gene discussed is PGAP3; the disease is Elevated circulating alkaline phosphatase concentration.