The viral thymidine kinases differ completely from the mammalian enzymes both structurally and biochemically and are inhibited by inhibitors that do not inhibit the mammalian enzymes. The genes of the two human isoenzymes were localized in the mid-1970s. The gene for TK1 was cloned and sequenced. The corresponding protein has a molecular weight of about 25 kD. Normally, it occurs in tissue as a dimer with a molecular weight of around 50 kD. It can be activated by ATP. After activation, is a tetramer with a molecular weight around 100 kD. However, the form of enzyme present in the circulation does not correspond to the protein as encoded by the gene: the main fraction of the active enzyme in the circulation has a molecular weight of 730 kD and is probably bound in a complex to other proteins. This complex is more stable and has a higher specific activity than any of the lower molecular weight forms.