The application relates to a composition comprising: a stably integrating delivery vector; a modified mammalian thymidylate kinase (tmpk) wherein the modified mammalian tmpk increases phosphorylation of a prodrug relative to phosophorylation of the prodrug by wild-type human tmpk; and a detection cassette fused to tmpk. The application also relates to use of these compositions in methods of treatment of diseases such as graft versus host disease and cancer.