The HSV-tk/ganciclovir gene suicide system was shown to be associated with cell cycle arrest in the G1/S-phase: Zeng et al. reported that after treatment of a breast cancer cell line with a Tet-on-HSV-tk/GCV system, an increment of the S-phase status was noticed along with increased expression of CDKN1a and decreased expression of CDK1, CCNB and PCNA, suggesting that HSV-tk/GCV’s modus operandi includes cell cycle inhibition (31). The gene discussed is PCNA; the disease is breast cancer.