Abstract Hematopoietic stem cells (HSCs) sustain lifelong production of all blood cell types through finely balanced divisions leading to self-renewal and differentiation. Although several genes influencing HSC self-renewal have been identified, to date no gene has been described that, when activated, enhances HSC self-renewal and, when activated, promotes HSC differentiation. We observe that the retinoic acid receptor (RAR)? is selectively expressed in primitive hematopoietic precursors and that the bone marrow of RAR? knockout mice exhibit markedly reduced numbers of HSCs associated with increased numbers of more mature progenitor cells compared with wild-type mice. In contrast, RARa is widely expressed in hematopoietic cells, but RARa knockout mice do not exhibit any HSC or progenitor abnormalities. Primitive hematopoietic precursors overexpressing RARa differentiate predominantly to granulocytes in short-term culture, whereas those overexpressing RAR? exhibit a much more undifferentiated phenotype. Furthermore, loss of RAR? abrogated the potentiating effects of all-trans retinoic acid on the maintenance of HSCs in ex vivo culture. Finally, pharmacological activation of RAR? ex vivo promotes HSC self-renewal, as demonstrated by serial transplant studies. We conclude that the RARs have distinct roles in hematopoiesis and that RAR? is a critical physiological and pharmacological regulator of the balance between HSC self-renewal and differentiation. JEM ? The Rockefeller University Press.