The In vitro Mammalian Cell Gene Mutation Test can be used to detect the potential of a chemical to induce gene mutations. The hypoxanthine phosphoribosyl transferase (HPRT) test detects mutations in the endogeneous hypoxanthine-guanine phosphoribosyl transferase gene (Hprt in rodent cells, HPRT in human cells); more specifically, base pair mutations, frameshift mutations and small deletions and insertions. To estimate the mutagenic potential of a substance, treated cells are placed under selective pressure so that only mutant cells can survive. Lack of HPRT activity results in resistance to the toxic analogue 6-thioguanine (6TG). Thus, mutant cells are able to proliferate in the presence of TG, whereas normal cells, which contain HPRT, are not. The most commonly used cells for the HPRT test include the CHO, CHL and V79 lines of Chinese hamster cells, L5178Y mouse lymphoma cells, and TK6 human lymphoblastoid cells.