Expression of a 3′RR-deleted allele in B1 B cells of the spleen, peritoneal cavity, and fetal liver. (A) Flow cytometry analysis of IgM^a and IgM^b on spleen B1 B cells of a^Δ3′RR/b^wt and a^wt/b^wt mice. One representative experiment is shown. (B) IgM^a/IgM^b ratios for spleen B1 B cells. Six a^Δ3′RR/b^wt and nine a^wt/b^wt mice were investigated. Results are reported as means ± standard error of the mean (SEM), Mann-Whitney U test for significance. (C) IgM^a and IgM^b on spleen B2 B cells of a^Δ3′RR/b^wt and a^wt/b^wt mice. One representative experiment is shown. (D) IgM^a/IgM^b ratios for spleen B2 B cells. Same mice as in panel B. (E) IgM^a and IgM^b on peritoneal cavity B1 B cells of a^Δ3′RR/b^wt and a^wt/b^wt mice. One representative experiment is shown. (F) IgM^a/IgM^b ratios for peritoneal cavity B1 B cells, 7 mice per groups. (G) IgM^a and IgM^b on peritoneal cavity B2 B cells of a^Δ3′RR/b^wt and a^wt/b^wt mice. One representative experiment is shown. (H) IgM^a/IgM^b ratio for peritoneal cavity B2 B cells, 7 mice per groups. Same mice as in panel E. (I) IgM^a and IgM^b on B1 B cells in fetal liver (12 hours before birth) of a^Δ3′RR/b^wt and a^wt/b^wt mice. One representative experiment is shown. (J) IgM^a/IgM^b ratio for B1 B cells in the liver 12 hours before and 12 hours after birth. Eleven a^Δ3′RR/b^wt and nine a^wt/b^wt mice were investigated before birth. Twelve a^Δ3′RR/b^wt and seven a^wt/b^wt mice were investigated after birth.