The hematopoietic hierarchy. Hematopoiesis is organized in a hierarchical manner, with rare HSCs at the top that give rise to various types of progenitor cells, which proliferate extensively, finally generating mature blood cells at the bottom of the hierarchy. Red arrows indicate stimulation of proliferation by TGF-β, whereas inhibition signs point to TGF-β’s growth inhibitory effect, in specific cell types or lineages. CLP, common lymphoid progenitor; CMP, common myeloid progenitor; GMLP, granulocyte-macrophage-lymphocyte progenitor; GMP, granulocyte-macrophage progenitor; LT-HSC, long-term HSC; MEP, megakaryocyte-erythrocyte progenitor; MPP, multipotent progenitor; NK, natural killer; ST-HSC, short-term HSC.