The percent usage of RF1 and RF2 among unique, in-frame Igμ transcripts with recognizable DFL16.1 sequence from Hardy fractions B (CD19+ CD43+ IgM− BP-1−), C (CD19+ CD43+ IgM− BP-1+), D (CD19+ CD43− IgM− IgD−), E (CD19+ CD43− IgM+ IgD−), and F (CD19+ CD43− IgMlo IgDhi) from wild type, ΔD-DFL, and ΔD-DμFS mice is displayed. When compared to ΔD-DFL and wild type controls, the use of hydrophobic and neutral DH reading frames has been flipped in B lineage cells from ΔD-DμFS mice.