Fanconi anemia (FA) is a genetic syndrome predisposing to hematopoietic failure. Little is known about the pathophysiology of FA, except that tumor necrosis factor-α (TNF-α) is overexpressed in patients. FA group C (Fac) gene knockout mice have been developed in order to model the human disease, but the mice do not spontaneously exhibit aplasia. To investigate secondary influences on hematopoiesis in the Fac-null mice, we studied the sensitivity of hematopoietic progenitor cells (HPC) to death receptor triggering by TNF-α and Fas receptor (CD95) ligation. Previously we had found that overexpression of a human FAC transgene protects hematopoietic progenitors from Fas-mediated apoptosis (Wang et al., 1998, Cancer Res 58: 3538–3541). In the present experiments with Fac-null mice, growth of erythroid burst-forming units (BFU-E) was significantly inhibited by TNF-α and CD95 ligation. Flow cytometric analysis revealed that CD95 was induced more readily in the Fac-null CD34+ cell fraction. Apoptosis induced by TNF-α alone or with CD95 ligation also occurred more frequently in null mouse HPC. We then bred null mice against transgenic mice overexpressing TNF-α (at serum levels in the range of 100 pg/ml). Resultant Fac-null mice that overexpressed TNF-α not only yielded decreased numbers of BFU-E but also expressed higher levels of CD95 in the CD34+ fraction. We conclude that mutation in the Fac protein induces heightened sensitivity to TNF-α and Fas receptor ligation, results that may explain the mechanism of anemia in FA-C patients. J. Cell. Physiol. 179: 79–86, 1999. Published 1999 Wiley-Liss, Inc.