Deletions occur within the HSV-tk gene. (A) Agarose gel electrophoresis of PCR performed on DNA of an integral transgene. The 9 PCRs were performed on DNA extracted from 100% G1TKSvNa-transduced HUT cells. As expected, PCR W-04 produced 2 PCR products corresponding to the full-length and truncated HSV-tk gene. (B) Different deletion profiles (WP-01 to WP-04) obtained from clones of different patients. The absence of PCR products in successive PCRs delimited the deletion area. MW indicates molecular weight marker 100-bp DNA ladder. (C) Schematic representation of deletion areas on the G1TKSvNa vector linear map.