Failure to adequately use positive control data leads to poor quality mouse lymphoma data assessments.

The mouse lymphoma L5178Y/TK1/–-3.7.2C mutagenesis assay (MLA) is widely used to identify chemicals that are capable of inducing mutational damage (Mitchell et al., 1997). Mutants detected in the assay form a bimodal distribution based on colony size. More than two decades of research demonstrate that both small and large colony trifluorothymidine-resistant (TFTr) colonies are ‘true mutants’ and that the assay detects the broad spectrum of genetic damage known to be involved in the etiology of cancer (Moore-Brown et al., 1981; Moore et al., 1985b; Applegate et al., 1990; Hozier et al., 1992). Because the assay detects mutagens that act through a variety of mechanisms, the MLA is specifically recommended by regulatory agencies for inclusion in hazard identification screening for environmental chemicals and pharmaceuticals (Dearfield et al., 1991; ICH3, 1996). Because of the ability of the MLA to detect both point mutations and chromosomal mutations, it would be expected that some chemicals would be positive in the MLA but not in the Salmonella assay, which only detects chemicals capable of inducing point mutations. In addition, it would be expected that the MLA would yield mutant frequencies (including the spontaneous background mutant frequency) far higher than those characteristic of other gene mutation assays, such as hypoxanthine guanine phosphoribosyl transferase deficiency and ouabain resistance, which by their nature are unable to detect the same broad spectrum of mutational events (Moore et al., 1989). In fact, mutant frequencies .1000310–6 have been induced by a number of chemicals (Clive et al., 1979; Moore et al., 1989; Backer et al., 1990; Moore and Doerr, 1990). A properly conducted MLA includes the demonstration that both small and large colony mutants have been optimally quantitated. This is accomplished by the appropriate use of the positive control. Chemicals that have been successfully used as positive controls (in the absence of exogenous activation) include methyl methanesulfonate (MMS), ethyl methanesulfonate (EMS) and hycanthone. Mutant frequencies for these chemicals (and colony sizing information) were published in