Exotic mice for aging: Authors' reply

The authors reply here to comments made by Drs. McClearn and Festing about the strengths and weaknesses of nonstandard mouse models in experimental gerontology, stressing once again the trade-off between internal and external validity. Although the use of genetically isogenic mice may, in some cases, minimize experimental costs by diminishing variance among individuals, reliance on a small number of inbred, long-domesticated stocks diminishes the external validity of a research program, i.e., the degree to which its conclusions give insights into questions of greater biological interest than the behavior of a single genotype.