... 5¢-hydroxyl group of thymidine, forming dTMP. Human TK1 iscytosolic and its activity is absent in resting cells, appears in late G1,increases in S phase coinciding with the increase in DNA synthesis, ... the non-TK1 like kinases deoxycytidinekinase [15], deoxyguanosine kinase and Drosoph-ila melanogaster multi-substrate kinase [16]. This indi-cates a different evolutionary origin of the twoclasses ... cycle, in close correlationwith the rate of DNA synthesis, with low dNTP levels in G1cells increasing during S phase, generally withdTTP being the most abundant and dGTP the least[4–6]. In quiescent...