TheDrosophila melanogasterdeoxyribonucleoside kinase (Dm-dNK) double mutant N45D⁄N64D was identified during a previous directed evolution study. This mutant enzyme had a decreased activity towards the natural substrates and decreased feedback inhibition with dTTP, whereas the activ-ity with 3¢-modified nucleoside analogs like 3¢-azidothymidine (AZT) was nearly unchanged.