HSVtk gene expression level in implanted tumors of BALB/c nude mice and detection by RT-PCR. Total RNA was isolated from implanted tumors of nude mice.HSVtk gene expression level was detected by RT-PCR with the nested primers TK3/TK4 (a DNA fragment of 420 bp). For internal control, β-actin was amplified together with HSVtk. The amplification product of HSVtk gene was observed in the Dox+GCV+virus group and the weak signal was detected in GCV+virus group. M: 100 bp Marker; 1: DMEM group; 2: GCV+Dox group; 3: GCV+virus group; 4: Dox+GCV+virus group.