A,B, Flow cytometry analysis of H-chain allelic exclusion in b12H B cells. b12H mice (IgHb) were crossed to IgHa/a mice (“b12H/a”) to allow identification of cells carrying H-chains encoded by the endogenous allele and compared to B6 (b/b) or (B6 x IgHa)F1 (a/b) cells. A, Plot shows expression of IgMa and IgMb on B220+-gated splenocytes from the indicated strains. B, Statistical analysis of IgMb frequency among eight b12H/a and five a/b mice. ****, p < 0.0001.