Regulators of human HSCs. Genes and pathways regulating human HSC function have been identified using xenograft models. Self-renewal, proliferation, differentiation, and apoptosis of human HSCs are regulated by the interplay of these stimulatory and inhibitory modulators. AhR, aryl hydrocarbon receptor; SR1, StemRegenin; Angptls, angiopoietin-like proteins; Hh, hedgehog; PGE2, prostaglandin E2.