“…Although the HPRT mutation assay was developed originally for use in humans, animal studies are necessary to investigate the impact of genotoxic agents on the biological processes governing T‐cell development, kinetics, and mutation (Crippen & Jones, 1989; Walker et al, 1999, 2009; Walker & Meng, 2000). Thus, mutation assays based upon the cloning of Hprt ‐negative (6‐thioguanine resistant) T‐cells have been adapted to the mouse, rat, and monkey, with studies directed at defining quantitative aspects of mutation induction and expression time as well as qualitative characteristics like mutational spectra in both mice and rats (Aidoo et al, 1991, 1993, 1997; Casciano et al, 1999; Harback et al, 1999; Jones et al, 1985; Jones, Burkhart‐Schultz, & Crippen, 1987; Jones, Burkhart‐Schultz, Strout, & Crippen, 1987; Meng et al, 2004; Skopek et al, 1992; Tates et al, 1994; Walker et al, 1999; Zimmer et al, 1991).…”