UCK1 is a nucleoside kinase that catalyses the phosphorylation of the pyrimidine ribonucleosides uridine and cytidine, producing uridine monophosphate (UMP) and cytidine monophosphate (CMP). It can use either ATP or GTP as the phosphate donor. Its substrate range is narrow: it does not phosphorylate deoxyribonucleosides or purine ribonucleosides, which distinguishes it from kinases acting on those nucleoside classes.

Beyond the natural substrates, the enzyme can also phosphorylate a number of uridine and cytidine analogues. The annotated examples include 6-azauridine, 5-fluorouridine, 4-thiouridine, 5-bromouridine, N(4)-acetylcytidine, N(4)-benzoylcytidine, 5-fluorocytidine, 2-thiocytidine, 5-methylcytidine and N(4)-anisoylcytidine. This makes the UCK1 protein a useful subject for studies of how modified nucleosides are converted to their monophosphate forms.

The UCK1 gene is classed by HGNC in the uridine-cytidine kinase family, and UniProt places the protein in the uridine kinase family. NCBI Gene reports that alternative splicing gives rise to multiple splice variants encoding distinct isoforms. The enzyme carries the EC number 2.7.1.48 and has also been referred to as URK1. A mouse ortholog, Uck1, is recorded, which supports comparative work in rodent systems.