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All lymphocytes are derived from hematopoietic stem cells (HSC). The interleukin-7 receptor (IL-7R) transduces non-redundant signals for both T and B-cell development from HSC. The upregulation of the IL-7R occurs at the stage of the clonogenic common lymphoid progenitor, a recently identified population that can give rise to all lymphoid lineages (T, B and natural killer cells) at a single cell level. The IL-7R plays a critical role in the rearrangement of immunoglobulin heavy chain genes required for B-cell development. IL-7R expression is critically regulated in developing thymocytes; thymocytes that fail the positive selection process downregulate the IL-7R, but those undergoing positive selection upregulate or maintain IL-7R expression. Recent data indicate that IL-7 signaling enhances the survival of developing thymocytes and mature T cells, presumably by its upregulating Bcl-2. Detailed analysis of the signaling cascades activated by the IL-7R may help to reveal the differential roles of IL-7 signaling in T and B-cell development.