Despite the tremendous diversity of the cells of the hematopoietic system, they are all derived from common precursor cells that are generated in the fetus and persist into adult life. In this regard, Band T lymphocytes, which comprise the two arms of the antigen-specific and inducible immune system, though functionally very different, are descendants of the same stem cell precursor. In the past several years, we have witnessed an explosion of information regarding the process by which differentiation of B-and T-cells from stem cells occurs. This information, like the answers to most...