Cytosolic deoxycytidine kinase (DCK) catalyzes the reactions of cytidine or deoxycytidine with ATP to form CMP or dCMP and ADP. The enzyme is a homodimer (Sabini et al. 2003). Although a chimeric deoxycytidine kinase - green fluorescent protein expressed at high levels in cultured cells localized to nuclei, endogenous protein is primarily cytosolic (Hatzis et al. 1998). Despite its name, the enzyme has a broad substrate specificity, acting on cytidine, deoxycytidine, deoxyguanosine, and deoxyadenosine (Bohman and Eriksson 1988; Datta et al. 1989a, b; Sarup et al. 1989; Usova and Eriksson 2002). While ATP functions efficiently as a phosphate donor, other nucleoside triphosphates, notably UTP, function efficiently as phosphate donors in vitro and may function in this way in vivo as well (Shewach et al. 1992).