Although viral thymidine kinases are known to accept a broad spectrum of nucleoside analogs , the tested TK displayed a narrower substrate spectrum compared to DmdNK or HsdCK (Figure ). The screening mainly revealed activities towards deoxy-or arabinonucleoside substrates. Not only pyrimidines but also some purine nucleoside analogues were converted by TK, although to a much lower extent than by DmdNK and HsdCK. Interestingly, unlike DmdNK and HsdCK,