Regulating HSC progenitors via cholesterol Atherosclerosis is characterized by the buildup of cholesterol-containing lipoproteins in the vascular wall. This increased cholesterol augments hematopoietic stem and progenitor cell (HSPC) counts, and the resultant increase in leukocytes is associated with increased cardiovascular disease. Gu et al. describe a mechanism orchestrating HSPC specification from the hemogenic endothelium (HE) during embryogenesis (see the Perspective by Rajan and Berman). ApoA-I binding protein accelerated cholesterol efflux from the HE, activating the transcription factor Srebp2, which in turn transactivated Notch signaling. This mechanism also appears to be important for adult HSPC expansion in hypercholesterolemia. Science , this issue p. 1085 ; see also p. 1041