1997). In addition, recent analysis revealed that some description of the genetic background of the animals commonly used ES cell lines are polymorphic at a numstudied. This description should be exact and include ber of loci, showing that they were not derived from enough detail to allow rederivation of the mice used. inbred strains (Simpson et al., 1997). This raises the Second, the genetic background chosen should not be possibility that random segregation of these polymorso complex as to preclude others from reproducing and phic loci to either mutants or controls could affect the expanding the experiments reported. Third, use of a phenotypes of the resulting animals and complicate the common genetic background would facilitate the cominterpretation of experiments. Nevertheless, it is still parison of results across experiments and among labopossible to use the 129 ES cell lines currently available ratories. For a variety of reasons described below, we without compromising experiments, because congenic recommend that mutations be maintained in congenic mutant lines can be generated by backcrossing to stanlines, and that mutants be analyzed in a defined hybrid dard inbred mice. The extent of backcrossing required (and preferably F1) genetic background. will depend on the degree of polymorphism. Genetic