For example, no mutations were detected in the hypoxanthine-guanine phosphoribosyltransferase (Hprt) gene in Chinese hamster V79 cells (Glatt et al. 1992) and the thymidine kinase (tk) locus in mouse L5178Y/TK+/– lymphoma cells (Wangenheim and Bolcsfoldi 1988) in the absence of S9. The gene discussed is HPRT1; the disease is lymphoma.