High levels of Id2 expression inhibit neutrophil differentiation in vitro and promote myeloid progenitor proliferation in vitro and in vivo. (A) 5FU-BMCs were transduced with MSCV or MSCV-Id2 retrovirus and cultured in mGM-CSF to induce neutrophil differentiation. Myeloid cells in the culture were analyzed for c-kit, Sca-1, Gr-1, and Mac-1 surface markers. (B) Total cell numbers of Gr-1⁺Mac-1⁺ cells and c-kit⁺ cells from MSCV- or MSCV-Id2–transduced 5FU-BMC culture. *P < .05 in 2-sided t test. (C) Cell-cycle analysis of MSCV- or MSCV-Id2–transduced 5FU-BMCs was analyzed by propidium iodide staining. (D) Myeloid development in recipient bone marrow and spleen was examined 5 months after transplantation by FACS analysis with the use of Gr-1, and Mac-1 antibodies. Total cell number of Gr-1⁺Mac-1⁺ cells was shown. *P < .05 in 2-sided t test. Four recipient mice were used in each group. Three independent experiments were performed.