Deoxyribonucleoside kinases catalyze the transfer of a phosphoryl group from ATP to a deoxyribonucleoside,a key step in DNA precursor synthesis.Furthermore,they also display a very broad specificity as regards nucleoside acceptors,a fact widely profited from the development of antiviral and antitumour nucleoside analogues,the activities of which are dependent on their prior intracellular phosphorylation by nucleoside kinase.