How pure are inbred strains of mice?

As useful as immunologists find the inbred mouse, they cannot help but become disenchanted when they encounter genetically disparate mice in a purportedly inbred strain. Genetic impurity (allogenicity) can arise from a number of sources: incomplete inbreeding, mutation, inadvertent outcrossing, mislabeling, and epislatic and heterozygote selection. In this article D. W. Bailey discusses these vitiating sources and the appropriate precautions against there.