While the acceptance of dAdo, dCyd and dGuo has already been described several times in the literature , reports on the acceptance of ribonucleotides are rare. Yet, the use of cytidine as substrates for human T-lymphoblast deoxycytidine kinase has been described before. [30] In this study, we also observed a minor accumulation of TMP (~13 %, HPLC analysis) and AMP (~1 %, HPLC analysis) with HsdCK as biocatalyst (Figure ). There have been many reports that HsdCK does not accept thymidine, and it is believed that phosphorylation of this nucleoside in vivo is carried out by human thymidine kinase 1 (TK1). However, kinetic studies on wild type and mutant HsdCK revealed that thymidine is accepted with low rate constants , which correlates well with observations made in this study. The acceptance of adenosine as substrate, however, has not been described so far. Although binding of adenosine to HsdCK was shown , AMP forming activity has never been reported. Most probably AMP appears due minor degradation of ATP during the kinase reaction, as low AMP formation was also observed in controls lacking enzymes (data not shown).