During routine genetic screening of several immunoglobulin heavy chain congenic mouse strains in 1980, one of us (MB) was surprised to find that several mice in the C.B-17IIcr strain, which was being maintained in a specific-pathogen-free facility of the Fox Chase Cancer Center (Philadelphia, PA), did not express serum immunoglobulin of the appropriate allotype. Fearing an error in the breeding of these mice, the sera of the suspect mice were screened for other allotypes. When these tests revealed a complete absence of serum immunoglobulin, it became apparent that a mutation had probably...