Backcross for a^Δ3′RR/b^wtand a^wt/b^wtmice, and membrane expression of IgM^aor IgM^ballele by B cells. The equilibrium between IgM^a- or IgM^b-expressing B cells will be disrupted if the expression of the 3′RR-deleted allele impedes B-cell development. Lowered number of IgM^a-expressing B cells in a^Δ3′RR/b^wt mice thus will demonstrate that deletion of the 3′RR alters B-cell development or recruitment.