Genetically heterogeneous (GH) mice and rats continue to be widely used in research even though the case for using isogenic strains has been argued repeatedly. The paper by Miller et al. in this issue appears to be the only one in the last 22 to attempt a scientific justification for the continued use of (a limited subset of) GH stocks. However, although they are to be commended for bravery, they fail to make their case. GH stocks represent poor material for controlled studies because genetic heterogeneity normally leads to phenotypic variability and a decline in experimental sensitivity. To counter this argument, Miller et al. claim that phenotypic variability may actually be smaller in GH animals than in their isogenic parents. Were this so (e.g., all mice being short lived, small, and aggressive), it is difficult to see how the use of such a stock could increase the generality of research results based on it, as claimed by Miller et al. Isogenic strains are a vital, proven, and powerful resource for biomedical research, and should be used in preference to GH stocks by all scientists who use laboratory rodents.