Overexpression of Id3 inhibits differentiation of B cells from CD34⁺CD38⁻CD10⁻ FL uncommitted precursors but does not affect development into monocytes. CD34⁺CD38⁻CD10⁻ FL cells were sorted, incubated with retroviral supernatant harboring the Id3-IRES-GFP sequence, and subsequently cocultured with S17 stroma. After 18 days, the cells were harvested and expression of B-cell (A) and monocyte (B) antigens was analyzed by flow cytometry. Thirty-two percent of the cells were GFP⁺ 2 days after transduction. A representative experiment of 3 performed is shown.