TK1 and cancer: The rationale supporting these strategies is, as follows: first, because of the high level of shared homology between the VV-tk gene and the human thymidine kinase 1 (TK1) gene, VACV lacking a functional tk gene can efficiently replicate in cancer cells, which often express high levels of TK1, a human cytosolic enzyme which plays the primary role in regulating intracellular thymidine pools throughout the cell cycle.