The HB-LTC CD19⁺ VpreB⁺population is composed of predominantly large, cycling cells. B-lineage cells were separated from 8-week-old HB-LTC by positive selection using anti-CD19 magnetic beads and stained with anti-VpreB and anti-IgM antibodies and analyzed by flow cytometry. (A) (Left) Forward and side scatter analysis of total cell population showing gated R1 (smallest) or R2 (largest) cell populations. (Right) VpreB⁺ and VpreB⁻ populations in gates R1 or R2, with numbers indicating the percentages of the total CD19⁺ cell population in each quadrant. (B) Cell cycle analysis of CD19⁺VpreB⁺ cells. CD19⁺ were stained with the anti-VpreB antibody before permeabilization and staining with propidium iodide for FACS analysis of DNA content versus surface VpreB expression (R3 = VpreB⁻ and R4 = VpreB⁺ populations, respectively). Numbers indicate the relative percentage of cells in S/G2/M.