STAT5, ID proteins, and the MPD phenotype. Schematic illustration of erythrocyte formation from HSCs showing the various levels at which ID proteins act to enhance erythrocyte formation. HSC indicates hematopoietic stem cell; CMP, common myeloid progenitor cell; CLP, common lymphoid progenitor cell; MEP, megakaryocyte-erythrocyte progenitor cell; CFU-E, erythroid colony-forming unit; CFU-Mk, megakaryocyte colony-forming unit; I, ID1 enhances HSC self-renewal^34,35 ; II, ID2 enhances myeloid differentiation over lymphoid differentiation⁵⁴ ; III, ID2 enhances erythroid over megakaryocytic differentiation⁵⁴ ; and IV, ID1 enhances erythroblast survival (this paper).