RUNX1 DN-like mutant induces increased genomic instability compared with RUNX1 haploinsufficiency. AII_1 and AII_2 are iPSC clones of 2 patients from pedigree A; D(a) is 1 clone of 1 patient from pedigree D; AII_1_RUNX1 is the iPSC clone AII_1 overexpressing WT RUNX1; D(a)_shRUNX1_1 is the iPSC clone D(a) after RUNX1 knockdown by shRUNX1_1; and C2 and C3 are control iPSC lines. (A-B) CD43⁺ cells and (C-E) CD34⁺CD43⁺ cells were sorted at day 14 of culture. (A,C) Representative photos are shown; nucleus is stained with 4,6 diamidino-2-phenylindole (DAPI; blue) and double-strain breaks (DSBs) are stained (A) with P-H2AX (red) or (C) with p53-BP1 (red). The number of (B) P-H2AX and (D) P53-BP1 foci per 50 counted cells is shown. One representative experiment of 4 is shown (P < .0001 for the increase in P-H2AX–positive cells; P = .0164 for the increase in P53BP1-positive cells in pedigree A). (E) qRT-PCR analysis of different p53-dependent genes in CD34⁺CD43⁺ cells. Data are normalized to PPIA transcript level and expression is compared with control C2. The histograms show 1 representative experiment of 3, each in triplicate. Error bars represent 95% CIs. (F-H) CD14⁺CD15⁺ cells were sorted at day 15 of culture. (F) Representative photos are shown; nucleus is stained with DAPI (blue) and DSBs are stained with P53BP1 (red). (G) The number of P53BP1 foci per 100 counted cells is shown. One representative experiment of 3 is shown (P = .0017 for the increase in P53BP1-positive cells in AII_1). (H) qRT-PCR analysis of different p53-dependent genes in CD14⁺CD15⁺ cells. Data are normalized to PPIA transcript level and expression is compared with control C3. The histograms show 1 representative experiment of 3, each in triplicate. Error bars represent 95% CIs. *P < .05; **P < .01; ***P < .001; Student t test.