StemRegenin 1 is a cell-permeable purine derivative that acts as an antagonist of aryl hydrocarbon receptor and promotes the self-renewal of hematopoietic stem cells (HSCs). Culture of HSCs with SR1 led to a 50-fold increase in cells expressing CD34 and a 17-fold increase in cells that retain the ability to engraft immunodeficient mice. In the absence of cytokines. StemRegenin 1 did not induce proliferation, and at concentrations above 1 µM, it inhibits proliferation.