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Highlights • Patient tumor subclones that survive chemotherapy acquire primitive cell traits. • HDAC inhibitors can reverse chemo-acquired stemness states and abolish self-renewal abilities. • Belinostat promotes stem cell differentiation and inhibits HDAC and MYC pathways. • A ‘one-two punch’, chemotherapy-HDAC inhibitor combination strategy reverses chemo-induced resistant phenotypes., Cancer cell phenotypes evolve during a tumor's treatment. In some cases, tumor cells acquire cancer stem cell-like (CSL) traits such as resistance to chemotherapy and diminished differentiation; therefore, targeting these cells may be therapeutically beneficial. In this study we show that in progressive estrogen receptor positive (ER+) metastatic breast cancer tumors, resistant subclones that emerge following chemotherapy have increased CSL abundance. Further, in vitro organoid growth of ER+ patient cancer cells also shows that chemotherapy treatment leads to increased abundance of ALDH+/CD44+ CSL cells. Chemotherapy induced CSL abundance is blocked by treatment with a pan-HDAC inhibitor, belinostat. Belinostat treatment diminished both mammosphere formation and size following chemotherapy, indicating a decrease in progenitor CSL traits. HDAC inhibitors specific to class IIa (HDAC4, HDAC5) and IIb (HDAC6) were shown to primarily reverse the chemo-resistant CSL state. Single-cell RNA sequencing analysis with patient samples showed that HDAC targets and MYC signaling were promoted by chemotherapy and inhibited upon HDAC inhibitor treatment. In summary, HDAC inhibition can block chemotherapy-induced drug resistant phenotypes with ‘one-two punch’ strategy in refractory breast cancer cells., Graphical abstract Image, graphical abstract