Early stages of B cell differentiation can be identified by the status of the Ig genes and by the cell surface markers CD34, CD19, and surface Ig (sIg). In stem cells and common lymphoid precursors, the Ig genes are in germ-line configuration. These cells express CD34 but not CD19 or sIg. In pro–B cells, cells committed to the B cell lineage, the first step in μ heavy chain gene rearrangement, D-to-J rearrangement, is occurring. All of the components of the pre–B cell receptor except μ heavy chain (the surrogate light chain proteins λ5 and VpreB, and the proteins that constitute the pre–B cell receptor signal transduction module, Igα and Igβ) can be found in the cytoplasm of these cells. Pro–B cells are positive for CD34 and CD19, but they are negative for sIg. Once an effective, in-frame VDJ heavy chain rearrangement has occurred, and the resulting μ heavy chaine light chain genes are in germ-line configuration. These cells express cytoplasmic μ heavy chain and cell surface CD19, but they are negative for CD34 and sIg. After successful rearrangement of a light chain gene, the conventional B-cell receptor can be expressed on the cell surface, and the cell becomes an immature B cell that is positive for CD19 and sIg and negative for CD34.