Orotidine 5'-phosphate decarboxylase (ODCase) is a dimeric enzyme that decarboxylates orotidine 5'-monophosphate (OMP) to form uridine 5'-phosphate (UMP), an essential step in the pyrimidine biosynthetic pathway. In mammals, UMP synthase contains two domains: the orotate phosphoribosyltransferase (OPRTase) domain that catalyzes the transfer of phosphoribosyl 5'-pyrophosphate (PRPP) to orotate to form OMP, and the orotidine-5'-phosphate decarboxylase (ODCase) domain that decarboxylates OMP to form UMP.