Several such tests exist, but the most frequently applied examples are the mouse lymphoma assay (MLA), which utilises the autosomal thymidine kinase (Tk) gene as a reporter of mutations in the L5178Y/Tk+/− mouse lymphoma cell line; and the X-chromosome linked hemizygous hypoxanthine guanine phosphoribosyltransferase (HPRT) gene forward mutation assay. Here, TKT is linked to lymphoma.