TKT and cancer: Consistent with their relative cancer cell specificity, Ad-ERE73 (1 + 2)-ARF (−13)-TK and ERE73 (3 + 4)-ARF (−13)-TK exhibited correspondingly higher cytotoxicity in pRB mutant cancer cell lines (5637 and Saos-2) compared to putative pRB wild-type cancer cell lines (DLD-1 and HLF) (Figure 3A).