Modified MSCs can express specific enzymes, in particular, HSV-thymidine kinase (TK) or cytosine deaminase (CD), converting inactive intravenously injected prodrugs, including ganciclovir (GCV) and 5-fluorocytosine (5-FC), into active cytotoxic agents, which in turn, can decrease systemic toxicity by easing tumor-localized chemotherapeutic activity (Pastorakova et al., 2020). Here, TKT is linked to neoplasm.