Orotic aciduria is a rare (fewer than 20 re- ported cases) inborn error of metabolism. Affected individuals exhibit retarded growth and develop- ment, as well as a megaloblastic anemia. The disease, which is autosomal recessive, is caused by a deficiency of the UMP synthase complex (phos- phoribosyltransferase and orotate decarboxylase) in the pyrimidine biosynthetic pathway (l'able 14- 2). The gene for UMP synthase is on the long arm of chromosome 3 (Table 16-5). Affected individu- als cxcrctc about 1.5 g of orotate daily; this is 1000 times the normal value. The anemia is due to a deficiency of nucleotides, precursors of DNA and RNA, in rapidly dividing bone marrow cells. Oral pyrimidines (synthetic or from yeast extract) pro vide substrates for pyrimidine nucleotide, RNA, and DNA synthesis to correct the anemia of orotic aciduria.