TKT and lymphoma: In this assay, Tk-deficient (Tk-/- or Tk0/-) mutants of the L5178Y/Tk+/- mouse lymphoma cells are selected by the pyrimidine analog trifluorothymidine (TFT) because TFT inhibits division of the Tk competent (Tk+/-) cells that are capable of incorporating TFT into the DNA.