Imetelstat inhibits the proliferation of MF stem and progenitor cells but has limited effects on their normal counterparts. Normal CB (A-C) or MF (D-F) CD34⁺ cells were treated with cytokines alone or cytokines plus increasing doses of an MM or imetelstat (Ime) for 7 days. Cells generated were phenotypically characterized and were assayed for HPCs. The percentage of the absolute number of Lin⁻CD34⁺ cells (A,D), all classes of assayable HPCs (B,E), as well as CD34⁺ cells with ALDH activity (C,F) generated in the cultures of normal or MF CD34⁺ cells exposed to cytokines plus MM or cytokines plus Ime relative to that generated in the cultures exposed to cytokines alone are shown. CB: 1.8 μM and 3.75 μM: n = 4; 7.5 μM: n = 6. P all > .05, MM vs Ime at each dose. MF: *P < .05, **P < .01, ***P < .001, MM vs Ime. n = 14 (9 splenic MF and 5 PB MF).