Cytosolic uridine-cytidine kinase 2 (UCK2) catalyzes the reactions of cytidine or uridine with ATP to form CMP or UMP and ADP (Greenberg et al. 1977; Van Rompay et al. 2001). Crystallographic data show the enzyme to be a tetramer (Suzuki et al. 2004). When it is expressed as a fusion construct with green fluorescent protein, the protein localizes primarily to the cytosol of transfected CHO cells (Van Rompay et al. 2003).