Although the efficacy and feasibility of HSV-tk have been validated, there are still several limitations of HSV-tk: (1) HSV-tk is derived from virus with potential immunogenicity, therefore it may compromise the survival of functionally modified T cells [49]; (2) HSV-tk requires activation by a prodrug (like GCV) that remains a crucial pharmacologic agent for the treatment of cytomegalovirus infection [50]; (3) Because interfering DNA synthesis to induce T cells death is a gradual progress, it takes more time to eliminate CAR-T cells by HSV-tk [47]. Here, TKT is linked to cytomegalovirus infection.