TKT and neoplasm: As the affinity of GCV to HSV-TK is about 1,000 times higher than that of cell TK, GCV is preferentially phosphorylated by exogenous HSV-TK in TK-positive tumor cells (Osaki et al., 1994); in other words, the HSV-TK/GCV system has little effect on normal tissues while performing its antitumor role.