The MCL-1/BCL-XL switch during erythropoiesis. In the bone marrow, hematopoietic stem cells (HSCs) differentiate into red blood cells via multiple successive differentiation stages. Immature stem and progenitor cells and early erythroid progenitors specifically require MCL-1 for survival. During later stages, BCL-XL takes over this function. MEP, megakaryocyte-erythroid progenitor.