Stage-specific aberrant expansion or differentiation block of distinct stem and progenitor populations in MDS. (A) Normal primitive long-term hematopoietic stem cells (LT-HSCs; red circles) have the capacity for self-renewal and give rise to a balanced number of multipotent progenitors (orange circles), common myeloid progenitors (CMPs; blue circles), granulocyte-monocyte progenitors (GMPs; purple circles) and megakaryocyte-erythrocyte progenitors (MEPs; green circles). (B) Low risk MDS LT-HSCs are cytogenetically abnormal (orange bars) with expansion of CMPs but normal GMP and MEP distribution. (C) High-risk MDS LT-HSCs are significantly expanded with increased GMP and slightly reduced MEP compartments. Status of multipotent progenitors in low- and high-risk patients has not been evaluated in this study.