Expression of thymidine kinase variants is a function of the replicative state of cells

The expression of different variants of thymidine kinase (TdR kinase), characterized by electrophoretic mobilities, is related to the replicative state of normal or transformed cultured cells rather than the developmental state of the tissue of origin. The form of thymidine kinase found in actively growing cultured cells, corresponding to the “fetal kinase” of embryonic tissue, migrates as a slow moving species with an Rf of 0.20 on acrylamide gels. The “adult kinase” found in adult tissue or other nongrowing cells migrates as a faster species with an Rf of 0.50 on acrylamide gels.