Cooperation of deregulated c-Myb and Sox4 with the loss of p15Ink4b in myeloid colony formation. (A) Northern blot analyses of c-myb and Sox4 expression in M1 cells during interleukin-6 (IL-6)–induced monocytic differentiation. Loading and integrity of analyzed RNAs were confirmed by hybridization with the gapd probe. (B) Cell-cycle distribution of cKit⁺ cells purified from p15Ink4b^wt/wt- and p15Ink4b^fl/fl-LysMcre BM infected with empty retrovirus (MIG), or retrovirus expressing c-Myb (cMyb), or Sox4 (Sox4) transcription factors. (C) Myeloid colony formation of cKit⁺ BM cells purified from targeted (fl/fl) or wt (wt/wt) mice infected with empty vector (MIG) or transcription factors-expressing (cMyb or Sox4) retroviruses. GFP⁺ cells were sorted and cultured in methylcellulose with myeloid-specific cytokines. Data are mean ± SD (error bars) from 2 independent experiments, each performed in triplicate.