Deoxycytidine Kinase (DCK) is a member of the DCK/DGK family. DCK exists as a homodimer and is localized to the nucleus. DCK is required for the phosphorylation of the deoxyribonucleosides deoxycytidine (dC), deoxyguanosine (dG), and deoxyadenosine (dA). DCK has broad substrate specificity, and does not display selectivity based on the chirality of the substrate. In addition, DCK is also an essential enzyme for the phosphorylation of numerous nucleoside analogs widely employed as antiviral and chemotherapeutic agents. DCK is clinically important because of its relationship to drug resistance and sensitivity.