Deletion of Flip suppresses macrophage differentiation. cKit⁺ hematopoietic stem cells from Flip^f/+ and Flip^f/d were isolated from bone marrow and seeded at 2.5 × 10⁵ cells/well followed by infection of retroviral vectors expressing GFP alone (A-B) or GFP and Cre (C-D). After the infection, the cells were differentiated in vitro to macrophages in 20% L929 medium. The total number of GFP⁺ cells (A,C) and the GFP⁺, CD11b⁻, F4/80⁻, the GFP⁺, CD11b⁺, F4/80⁻, and the GFP⁺, CD11b⁺, F4/80⁺ cells (B,D) were determined after 1, 2, and 5 days of differentiation. Data in panels B,D represents the mean ± SEM of 4 independent experiments.