Leukemogenic fusion proteins induce senescence in hematopoietic progenitors. (A) Bone marrow cells were isolated from 5-fluorouracil-treated mice, transduced with a retrovirus expressing empty vector, H-RAS V12, or a leukemogenic fusion protein, and stained for senescence-associated β-galactosidase. (B) Immunoblot analysis monitoring levels of p16^INK4a and H3K9 Ac in hematopoietic cells expressing empty vector, H-RAS V12, or a leukemogenic fusion protein. (C) Hematopoietic cells expressing empty vector, H-RAS V12, or a leukemogenic fusion protein were analyzed by comet assay under alkali conditions. (D) Quantification of comet tail length. Error bars represent SEM. (E) Immunoblot analysis monitoring levels of phospho-ATM and, as a control, total ATM in hematopoietic cells expressing empty vector, H-RAS V12, or a leukemogenic fusion protein. (F) BrdU incorporation assay in hematopoietic cells stably expressing either a nonsilencing (NS) siRNA or one of 2 unrelated siRNAs directed against ATM, and expressing empty vector, H-RAS V12, or a leukemogenic fusion protein. Error bars represent SEM. (G) Immunoblot analysis of phosphorylated p39 (P), total p38 (T), and p16^INK4a levels in hematopoietic cells expressing empty vector, H-RAS V12, or a leukemogenic fusion protein treated with dimethyl sulfoxide (DMSO) or SB203580.