found that the accumulation of phosphorylated GCV-TP in 20–80% bystander cells of human glioblastoma cells expressing HSV-TK occurred readily, but this accumulation was dependent upon the percentage of HSV-TK-expressing cells, as well as the concentration of GCV and the length of incubation (Rubsam et al., 1999). The gene discussed is TKT; the disease is glioblastoma.