Mammalian cells lack efficient means of salvaging pathway for pyrimidine bases. Salvage reactions convert the pyrimidine ribonucleotides uridine and cytidine and the deoxyribonucleosides thymidine and deoxycytidine to their respective nucleotides. Deoxycytidine is phosphorylated by deoxycytidine kinase, an enzyme that also phosphorylates deoxyguanosine and deoxyadenosine. Orotate phosphoribosyl transferase, an enzyme of de novo synthesis of pyrimidine nucleotides, can salvage orotic acid to OMP.