IL-7 is required for progression from the CD45R⁺CD19⁻ stage to the CD45R⁺CD19⁺ stage.

Lin⁻c-kit^Lo (A) or Lin⁻c-kit⁻ cells (B) were sorted to high purity and placed in serum-free, stromal cell–free cultures with the indicated combinations of cytokines. Yields of CD45R⁺CD19⁻ (▪) and CD45R⁺CD19⁺ (▨) cells per input precursor were calculated as described in the legend to Figure 1.