CD9 as a therapeutic target for pYSTAT5-driven leukemia. (A-B) JAK2^V617F transplantation with aCD9 in vivo treatment. (A) Experimental workflow. (B) Log₂ fold changes (aCD9 vs IgG) of HSC subpopulations and total BM cell numbers of (i) Ly5.2⁺ JAK2^V617F donors or (ii) Ly5.1⁺ WT NSG recipients (n = 4 per treatment, mean ± SEM, 4× 1.25 mg/kg IV IgG or aCD9 were applied). (C) Analysis of CD9 expression in CD34⁺CD38⁻ cells of (i) MPN^JAK2V617F+ (n = 4) and (ii) CML^BCR/ABL1+ (n = 6) patient and control (n = 4) BM. (D-E) In vitro treatment of control and patient BM cells either treated with IgG or aCD9 (2 µg/mL). XY plots showing CD9 levels and (D) CD34⁺CD38⁻ cell numbers or (E) Annexin-V levels of control (n = 4) and (i) MPN^JAK2V617F+ (n = 3) or (ii) CML^BCR/ABL1+ patient BM mononuclear cells (n = 6). (F) Serial plating assays of CML^BCR/ABL1+ patient (n = 6) BM either treated with IgG or aCD9. Fold changes of aCD9/IgG colony numbers in the first and second plating. (G) Serial plating assays of MPN^JAK2V617F+ patient (n = 5) BM either treated with IgG or aCD9. Fold changes of aCD9/IgG colony numbers in the first and second plating. Levels of significance were calculated using an unpaired Student t test (C), and a paired Student t test (F-G). Levels of significance and correlation were calculated using Pearson in panels D-E. *P < .05; ***P < .001.