UCK1 is a uridine-cytidine kinase that catalyzes the phosphorylation of uridine and cytidine to uridine monophosphate (UMP) and cytidine monophosphate (CMP) but not the phosphorylation of deoxyribonucleosides or purine ribonucleosides. This enzyme can also phosphorylate uridine and cytidine analogs and uses both ATP and GTP as a phosphate donor. Alternative splicing results in multiple splice variants encoding distinct isoforms. Recombinant human UCK1 protein, fused to His-tag at N-terminus, was expressed in E.coli.