Provided herein is a method of producing large amounts of the enzyme thymidylate kinase from multiple species for drug discovery purposes. Also provided herein are NMR methods of monitoring a reaction involving a thymidine kinase.

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.