Thymidine-5 -O-(alpha,beta-methylene)diphosphate (TMP-CP), a hydrolytically stable derivative of TDP, functions as an inhibitor of thymidine kinase (Ki = 23 uM). It serves as a precursor in the synthesis of alpha,beta-methylene-2 -deoxythymidine 5 -triphosphate, which is a non-cleavable substrate for DNA polymerase.