Based on sequence similarity, cytosolic uridine-cytidine kinase-like 1 (UCKL1 aka F538) is thought to function as a novel human uridine kinase/uracil phosphoribosyltransferase (Kashuba et al. 2002). The two human uridine kinases 1 and 2 (UCK1 and UCK2) characterised to date phosphorylate uridine/cytidine to uridine monophosphate (UMP)/cytidine monophosphate (CMP) using ATP as a phosphate donor (Greenberg et al. 1977, Van Rompay et al. 2001).