We have investigated the hematopoietic phenotype of mice with a hypomorphic mutation in the Brca2/Fancd1 gene (Brca2^Δ27/Δ27 mutation). In contrast to observations made in other Fanconi anemia (FA) mouse models, low numbers of hematopoietic colony-forming cells (CFCs) were noted in Brca2^Δ27/Δ27 mice, either young or adult. Additionally, a high incidence of spontaneous chromosomal instability was observed in Brca2^Δ27/Δ27 bone marrow (BM) cells, but not in Brca2^+/Δ27 or Fanca^−/− BM cells. Although Brca2^Δ27/Δ27 CFCs were not hypersensitive to ionizing radiation, a very severe hematopoietic syndrome was observed in irradiated Brca2^Δ27/Δ27 mice. Conventional BM competition experiments showed a marked repopulation defect in Brca2^Δ27/Δ27 hematopoietic stem cells (HSCs), compared to wild-type HSCs. Moreover, we have observed for the first time in a DNA repair disease model a very significant proliferation defect in Brca2^Δ27/Δ27 HSCs maintained in their natural physiological environment. The progressive repopulation of wild-type HSCs transplanted into unconditioned Brca2^Δ27/Δ27 recipients is reminiscent of the somatic mosaicism phenomenon observed in a number of genetic diseases, including FA. The hematopoietic phenotype associated with the Brca2^Δ27/Δ27 mutation suggests that this FA-D1 mouse model will constitute an important tool for the development of new therapies for FA, including gene therapy.