CMPK1 plays a key role in the maintenance of pyrimidine nucleotide pool profile and for the metabolism of pyrimidine analogs in cells. It catalyzes the phosphoryl transfer from ATP to UMP, CMP, and deoxy-CMP (dCMP), resulting in the formation of ADP and the corresponding nucleoside diphosphate. CMPK1 also has a significant role in the activation of pyrimidine analogs, which are clinically useful anti-cancer and anti-viral drugs. In the meanwhile, CMPK1 functions in cellular nucleic acid biosynthesis.