IRF-4 deficiency facilitates BCR/ABL transformation of B-lymphoid progenitors. Bone marrow from IRF-4 WT, IRF-4 Het, or IRF-4 KO mice was infected with MSCV retrovirus containing sequences for BCR/ABL-IRES-GFP or GFP, then triplicates of 2 × 106 cells were plated in soft agar media in the absence of cytokines. Colonies were counted after 10 days. Representative soft agar plates and quantitation of the corresponding colonies are presented in panel A and panel B, respectively.