OECD 490: Cell gene mutation tests using the thymidine kinase gene (MLA TK, in vitro mammalian)

The in vitro Mammalian Cell Gene Mutation Test is used to detect gene mutations induced by chemicals. The cell line L5178Y TK+/--3.7.2C mouse lymphoma is used for the assay to measure mutations in the thymidine kinase gene. Cells deficient in thymidine kinase (TK) due to the mutation TK+/- to TK-/- are resistant to the cytostatic effects of the pyrimidine analogue trifluorothymidine (TFT). 

Normal cells can produce the TK enzyme, therefore are sensitive to TFT, which causes the inhibition of cellular metabolism and prevents further cell division. Whereas mutant cells without the functional TK gene can proliferate in the presence of TFT. The mutant frequency is derived from the number of mutant colonies in the selective medium and the viability. The deficiency in TK can result from genetic events affecting the TK gene including both gene mutations (point mutations, frame-shift mutations, small deletions, etc.) and chromosomal events (large deletions, chromosome rearrangements and mitotic recombination).