Stages of B-cell development are defined by which gene segments are undergoing rearrangement as well as by cell-surface proteins. The earliest B-lineage surface markers are CD19 and CD45R (B220 in the mouse). The expression of these persists throughout B-cell development. A pro-B cell is also distinguished by expression of CD43, c-Kit, and the IL-7 receptor. An early pro-B cell is rearranging D to JH and a late pro-B cell VH to DJH. A late pro-B cell starts to express CD24 and CD25. When a late pro-B cell makes a successful VDJ join, it can then express an immunoglobulin heavy chain, which defines it as a pre-B cell. A pre-B cell is phenotypically distinguished by expression of BP-1, whereas c-Kit and the IL-7 receptor are no longer expressed. Before going on to rearrange a light-chain locus, a pre-B cell enlarges and undergoes several cycles of cell division. It then becomes a small pre-B cell, which can rearrange light-chain gene segments, first at the k locus and, if not successful, then at the l locus. A cell that has made a productive light chain becomes an immature B cell expressing surface IgM, and has completed the antigen-independent phase of B-cell development.