[MIM*258900] A rare disorder of pyrimidine metabolism characterized by hypochromic anemia with megaloblastic changes in bone marrow, leukopenia, retarded growth, and urinary excretion of orotic acid; autosomal recessive inheritance, caused by mutation in the uridine monophosphatate synthase gene (MMPS) on 3q13. [orotic acid + G. ouron, urine]