1.1   This guide highlights some of the more relevant biological concepts as they are currently understood, and summarizes the critical technical aspects for acceptable bioassay performances as they currently are perceived and practiced. The Chinese hamster ovary cell/hypoxanthine guanine phosphoribosyl transferase (CHO/HGPRT) assay (1)2 has been widely applied to the toxicological evaluation of industrial and environmental chemicals. The method is limited to detection of small-scale genetic interactions. Therefore, when this method is used for genotoxicity assessment it is recommended that an in vitro clastogenicity assay (for example, chromosomal aberration, micronucleus) is considered to detect large-scale (for example, chromosomal) DNA damage.