Abstract The synthesis and X-ray crystal structures of a series of 5-substituted-6-aza-2′- deoxyuridines is reported. These nucleoside analogues inhibit the phosphorylation of thymidine by HSV-1 TK but have no effect on the corresponding human enzyme. Detailed examination of one analogue proves it to be a competitive inhibitor of thymidine with a Ki of 0.34 μM and is a very poor substrate. The analogues are not substrates for the enzyme ...