Additional history obtained from the mother revealed significant family history: one maternal aunt died at seven years of age from a metabolic disorder of unknown etiology, while another maternal aunt was found by exome sequencing to carry a pathogenic heterozygous mutation in the X-chromosomal OTC gene, associated with OTC deficiency and hyperammonemia, which leads to one of the forms of hyperammonemia. The gene discussed is OTC; the disease is metabolic disease.