c-JUN is involved in p185^BCR-ABL-induced transformation and leukemogenesis. Colony formation assays were performed using 1 × 10⁶ (A) JNK1^+/− and JNK1^−/− (n = 8; 2-tailed t test, 49.2 ± 7.9 vs 28.9 ± 4.1 colonies/10⁷ fetal liver cells, P = .0401) and (B) c-Jun^fl/fl and c-Jun^fl/flCD19-Cre^+/− (n = 6; 2-tailed t test, 60.5 ± 6.1 vs 27.3 ± 1.9 colonies/10⁷ fetal liver cells, P = .0004) fetal liver cells after infection with a pMSCV-p185^BCR-ABL-IRES-GFP retrovirus in growth factor–free methylcellulose. (C) Transplantation of 1 × 10⁵c-Jun^fl/fl and c-Jun^Δ/Δ cells into Rag2^−/− mice. Two independent cell lines for each cell type were injected in 9 mice (mean survival, 11 vs 16 days in mice injected with c-Jun^fl/fl and c-Jun^Δ/Δ cells, respectively, P = .0307). (D) Spleen weights of diseased recipient Rag2^−/− mice were analyzed (c-Jun^fl/fl [n = 9] and c-Jun^Δ/Δ [n = 9], 2-tailed t test, P = .0169).