Stage-specific expansion in the early B-cell compartment. The figure displays a schematic drawing of early B-cell development indicating stage-specific recombination events of the immunoglobulin heavy-chain (IgHR) and light-chain (IgLR) genes and the expansion or deletion (apoptosis indicated by a cross) of early pre–B cells. The lower part of the figure indicates the specific stages of the cell cycle (G₁ [gap I], S [synthesis], G₂ [gap II], and M [mitosis]), as well as the cell-cycle block in G₁ induced by p21 and the ability of Wip1 to modulate the activity of p53 in G₂ phase.