We used a murine model containing a disruption of the murine homologue (Fac) of Fanconi Anemia group C (FAC) to evaluate the role of Fac in the pathogenesis of bone marrow (BM) failure. Methylcellulose cultures of BM cells fromFac^−/− and Fac^+/+ mice were established to examine the growth of multipotent and lineage-restricted progenitors containing inhibitory cytokines, including interferon-γ (IFN-γ), tumor necrosis factor-α (TNF-α), and macrophage inflammatory protein-1α (MIP-1α). Clonogenic growth of Fac^−/− progenitors was reduced by 50% at 50- to 100-fold lower concentrations of all inhibitory cytokines evaluated. We hypothesized that the aberrant responsiveness to inhibitory cytokines in clonogenic cells may be a result of deregulated apoptosis. To test this hypothesis, we performed the TUNEL assay on purified populations of primary BM cells enriched for hematopoietic progenitors or differentiated myeloid cells. After stimulation with TNF-α, accentuated apoptosis was observed in both populations of Fac^−/− cells. In addition, deregulated apoptosis was also noted in the most immature phenotypic population of hematopoietic cells after stimulation with MIP-1α.Together these data suggest a role of Fac in affecting the signaling of multiple cytokine pathways and support cytokine-mediated apoptosis as a major mechanism responsible for BM failure observed in FA patients.