Leukotriene C4 synthase deficiency is an extremely rare fatal neurometabolic developmental disorder characterized clinically by muscular hypotonia, psychomotor retardation, failure to thrive, and microcephaly. The disease is MONDO_0013539 (hypotonia-failure to thrive-microcephaly syndrome). Also known as: LTC4 synthase deficiency, leukotriene C4 synthase deficiency.