Human FA-C progenitor cells are hypersensitive to the mitotic inhibitory influence of IFN-γ. A parallel study of CD34⁺ cells from the BM of a normal volunteer (□) (67.5 ± 19 BFU-E/5,000 cells, 85.5 ± 6.3 CFU-GM/5,000 cells, and from a child with FA-C () (4 ± 1.4 BFU-E/5,000 cells, and 7 ± 4 CFU-GM/5,000 cells) are shown here. Both BFU-E (A) and CFU-GM (B) growth (mean ± SEM of replicate methylcellulose plates) were inhibited by doses of IFN-γ that have never suppressed clonal growth of normal human progenitor cells in more than 14 studies.