OTC and urea cycle disorder: Ornithine transcarbamylase deficiency (OTCD, OMIM #311250), the most common urea cycle disorder with an estimated incidence of 1:40,000–70,000, is caused by mutations in the X-linked OTC gene encoding the mitochondrial matrix homo-trimeric enzyme that catalyzes the synthesis of citrulline, an essential step to convert the neurotoxic ammonia into urea.