Cytosolic guanylate kinase 1 (GUK1) catalyzes the reversible reactions of GMP and dGMP with ATP to form GDP and dGDP respectively and ADP. While native gel electrophoretic studies of whole cell extracts suggested the existence of multiple human enzymes with guanylate kinase activity (Jamil et al. 1975), only one has been purified and biochemically characterized (Agarwal et al. 1978; Brady et al. 1996). The enzyme is of clinical importance as it is a target of antitumor drugs and antiviral drugs such as acyclovir (Miller and Miller 1980).