Quantitative stem and progenitor alterations in MDS subgroups. Stem cells and various progenitor populations are shown in normal and myelodysplastic hematopoiesis. Although this is a simplified model of differentiation, recent studies have shown that multipotent progenitors (MPPs) and CMPs can take alternative paths toward differentiation. Expansion of phenotypic HSCs is seen in MDSs and is most pronounced in higher-risk subgroups. Lower-risk MDS samples are characterized by phenotypic CMP expansions and decreased MEPs. Higher-risk samples are associated with GMP expansions.^1,4,12  RBC, red blood cell; ST-HSC, short-term hematopoietic stem cell.