Abnormal HSCs persist during remission, and their expansion occurs before clinical relapse. MDS9 patient has refractory anemia with excess blasts and attained remission with 5-azacytidine + vorinostat treatment (black arrows) with improvement of anemia (top blue line). Even when the patient was in remission, the HSC compartment was expanded (red line) and harbored cells with monosomy 7 (49% HSCs; FISH showed 97% of these had monosomy 7). Further expansion of HSCs (red arrow) occurred 2 months before relapse with increasing blasts and progressive anemia.