Effect of the 3′RR on BCR intensity and μ transcription in B1 B cells. (A) IgM^a and IgM^b intensities on the membrane of fetal liver B1 B cells. Results are reported as means ± SEM. Mann-Whitney U test for significance. Four a^Δ3′RR/b^wt and five a^wt/b^wt mice were investigated. NS: not significant. (B) IgM^a and IgM^b intensities on the membrane of peritoneal cavity B1 B cells. Seven a^Δ3′RR/b^wt and seven a^wt/b^wt mice were investigated. (C) μ transcription in sorted peritoneal cavity B1 B cells of 3′RR-deficient mice and wt mice. Four separate experiments with pooled peritoneal cells from 4 to 6 mice for each genotype. Values were normalized to GAPDH transcripts. Locations of PCR primers are indicated in the scheme. (D) κ transcription in sorted peritoneal cavity B1 cells of 3′RR-deficient mice and wt mice. Same mice as in panel C.