Hypersensitivity of Fac^−/−hematopoietic progenitors to inhibitory cytokines. Unfractionated BM cells from ^−/− (▪) and ^+/+ (•) animals were cultured in triplicate in clonogenic methylcellulose progenitor assays at 5 × 10⁴ cells/mL with increasing concentrations of IFN-γ (0.1, 0.5, 1, 5, and 10 ng/mL), TNF-α (0.1, 0.5, 1, 5, 10 ng/mL), or MIP-1α (1, 5, 10, 50 ng/mL). After 7 days in culture, CFU-GM, BFU-E, and CFU-GEMM were scored. Percentage maximal colony formation was determined by dividing the number of progenitors scored at a given concentration of cytokine by the number of progenitors scored without the addition of inhibitory cytokine. The range of control numbers upon which the percentage change are based:^+/+ CFU-GM 77-141, BFU-E 41-43, CFU-GEMM,11-13 and ^−/− CFU-GM 80-154, BFU-E 11-43, and CFU-GEMM.5-13 The error bars represent SEM. CFU-GM, BFU-E, and CFU-GEMM from Fac^−/−unfractionated BM cells were significantly more sensitive to IFN-γ, TNF-α, and MIP-1α at multiple cytokine concentrations. n = 4 for each group. *P < .05.