How disturbances of B cell development provoke adult B acute lymphoblastic leukemia (B-ALL) remains poorly understood. Here we describe Irf4–/– mice as prone to developing B-ALL with age. Irf4–/– pro/preB cells exhibited impaired differentiative but enhanced proliferative potential in vitro and accumulated in spleens of healthy Irf4–/– mice, suggesting reduced adherence to the IL-7 providing bone marrow niche. Thus selected, pro/preB cells transformed acquiring proliferative IL-7 independency through Jak3 gain-of-function mutations. Targeting JAK signaling with Ruxolitinib in vivo prolonged survival of mice bearing established Irf4–/– leukemia. Intriguingly, organ infiltration including leukemic meningeosis was selectively reduced without affecting blood blast counts. As low IRF4 expression and JAK3 mutations also characterize a subpopulation of Ph-like B-ALL in adult humans, our results imply Irf4–/– mice as a suitable model for investigating preleukemic conditions in adults. Using this model, we identified an unexpected effect of Ruxolitinib treatment in B-ALL.