TKT and cancer: RNA trans-splicing has been used to deliver the two-step Herpes simplex virus thymidine kinase/ganciclovir (HSV-tk/GCV) cell death system as a potential cancer therapy.20, 21, 22, 23, 24 HSV-tk acts by phosphorylating the pro-drug GCV, an analog of deoxyguanosine triphosphate, into an active compound, leading to chain termination during DNA replication and cell death (reviewed by Duarte et al25).