Thymidylate kinase fusions and uses thereof

A method of enhancing the capability of a mammalian cell in phosphorylating AZT comprising introducing into the cell in vitro or ex vivo a

• a stably integrating delivery vector, the vector comprising;
• i) a modified human thymidylate kinase (tmpk) polynucleotide that encodes a modified human tmpk polypeptide comprising a modification selected from (a) a F to Y mutation at amino acid position 105 of SEQ ID NO:
• 11, (b) a R to G point mutation at amino acid position 16 of SEQ ID NO:
• 12, or (c) a R to A mutation at amino acid position 200 of SEQ ID NO:
• 16 that increases phosphorylation of azidothymidine (AZT) relative to phosphorylation of AZT by wild type human tmpk polypeptide;
• ii) a detection cassette polynucleotide that encodes a detection cassette polypeptide that is cell surface expressed, wherein the detection cassette polynucleotide is fused to the modified human tmpk polynucleotide and the detection cassette polypeptide is fused to the modified human tmpk polypeptide;
• and
• iii) a promoter functional in a mammalian cell operably connected to the coding sequence of the tmpk/detection cassette fusion polynucleotide;
• wherein introduction of the delivery vector into the cell and expression of the modified human tmpk polypeptide enhances the capacity of the cell in phosphorylating AZT.