Mitochondrial thymidine kinase 2 (TK2) catalyzes the reactions of deoxycytidine, thymidine, and deoxyuridine with ATP to form the corresponding deoxynucleotide monophosphates and ADP. The enzyme has been purified from human spleen and is active as a monomer (Munch-Petersen et al. 1991). The enzyme requires divalent cations for activity (Mg++ is preferred) but the nature of the association between the metal ion and the enzyme polypeptide is unclear (Lee and Cheng 1976). The mitochondrial localization of the enzyme has been established experimentally for rats and cattle (Jansson et al. 1992); its mitochondrial localization in humans is inferred from these results and the presence of a mitochondrial localization motif at the amino terminus of the open reading frame of a cloned human cDNA that is missing from the mature catalytically active protein (Wang et al. 1999).