PARP1 promotes the growth of MLL-AF9–positive leukemias. (A) Clonogenic activity of Parp1^+/+ (+/+) and Parp1^−/− (−/−) mBMCs expressing MLL-AF9 (MSCV-MLL-AF9-IRES-GFP, green bars) or empty plasmid (MCSV-IRES-GFP, purple bars) as described in the supplemental Materials and supplemental Results. Results represent mean number of colonies per 10⁴ cells ± SD that formed after the second plating from 3 experiments. (B) Clonogenic activity of mBMCs expressing the indicated genes or empty plasmid (wild-type [wt]) and treated with increasing concentrations of olaparib for 72 hours following the plating in methylcellulose. Results represent mean percentage ± SD of surviving colonies compared with untreated counterparts from 3 experiments. (C) The effect of olaparib on Lin^–CD34⁺, Lin^–CD34⁺CD38^–CFSE^max, and Lin^–CD34⁺CD38^–CFSE^low cells obtained from AML primary cells expressing MLL-AF9 (3 patients, green symbols) and from healthy donors (3 donors, purple symbols). Results represent mean percentage ± SD of cells surviving after 5 days of treatment compared with untreated counterparts from 3 experiments per sample. *P = .03 when compared with the corresponding +/+ cells using Student t test.