The majority of early CD45R⁺ pro-B cells in BM are c-kit⁻. The boxed region of Panel A shows CD45R⁺CD43⁺ cells in lymphoid-enriched BM, from which BP-1⁻ CD25⁻ NK1.1⁻sIgM⁻ (B) cells were sorted. These gating criteria allowed isolation of early CD45R⁺ pro-B cells that were subsequently stained for surface c-kit, fixed, permeabilized, and stained for intranuclear TdT. Expression of c-kit (C) and TdT (D) in this costained population is shown. Panel E shows c-kit expression in the gated TdT⁺ cells within CD45R⁺CD43⁺ BP-1⁻ CD25⁻NK1.1⁻ sIgM⁻ cells. The frequencies of these subsets in BM are shown in Table 3. Means are from two independent experiments performed on pooled marrow from five or seven mice.