Most leukemias from Mb1-CreΔPB mice resemble pro-B cells and do not express either IgM or Igκ at the cell surface. (A) Representative flow cytometric analysis for the presence of CD19⁺ B220⁺ B cells in the spleen of C57BL/6 mice; 11-week-old Mb1-CreΔB mice and Mb1-CreΔPB mice (left panels); and >15-week-old Mb1-CreΔPB mice (right panel). (B) Representative flow cytometric analysis for the presence of CD19⁺ B220⁺ B cells in the thymus of C57BL/6 mice; 11-week-old Mb1-CreΔB mice, and Mb1-CreΔPB mice (left panels); and >15-week-old Mb1-CreΔPB mice (right panel). (C) Polymerase chain reaction for detection of heavy chain rearrangements (J558-JH4) in leukemia B cells prepared from the thymus of Mb1-CreΔPB mice. B cells prepared from WT mouse (C57BL/6) were used as control. The Cd79 gene was used as control for DNA quality. (D) Representative flow cytometric analysis of leukemic cells from Mb1-CreΔPB mice gated on CD19⁺ B220⁺ cells showed that leukemias expressed IL-7R on the cell surface (top and bottom). (E) Representative IL-7R⁺ leukemias expressing IgM and Igκ, (top, Ig⁺) and those not expressing IgM and Igκ (bottom, Ig⁻). (F) Percentage of leukemias expressing IgM and Igκ (Ig⁺) or not expressing IgM and Igκ (Ig⁻). (Left) Mb1-CreΔPB mice, n = 14 (spleen) and n = 8 (thymus). (Right) CD19-CreΔPB mice, n = 9 (thymus).