MDS BM shows expanded stem and progenitor populations. (A) Representative samples of lower- and higher-risk MDS and a healthy control. Shown are FACS analyses of anti-CD34/CD38 costainings within CD34-enriched, viable, lineage marker-negative BM mononuclear cells. (B) Quantification of phenotypic HSCs in healthy control patients (N = 16), lower-risk (n = 8), and higher-risk (n = 9) MDS patients showing a significant expansion of the HSC compartment in patients with higher-risk MDS compared with controls (P < .05, t test). (C) Quantification of phenotypic LT-HSCs and ST-HSCs in healthy control patients (N = 16), lower-risk (n = 8), and higher-risk (n = 9) MDS patients. *P < .05 (t test). **P < .005 (t test). (D) Representative samples of 1 lower-risk and 2 higher-risk MDS patients and a healthy control patient. Shown are FACS analyses of CD123 and CD45RA expression on viable, lineage marker-negative CD34⁺CD38⁺ BM mononuclear cells defining myeloid populations: red represents CMP; blue, MEP; and green, GMP. (E) Quantification of phenotypic myeloid progenitors in healthy control patients (N = 16), lower-risk (n = 8), and higher-risk (n = 9) MDS patients showing a significant expansion of the CMP compartment in patients with lower-risk MDS, and significant expansion of the GMP compartment, and significant reduction of the MEP compartment in higher-risk MDS. *P < .05 (t test). **P < .005 (t test).