Mice homozygous for a truncating mutation in Brca2 surviving to adulthood die from thymic lymphoma. BRCA2 regulates RAD51 recombinase which is essential in dividing cells. Mice carrying a T-cell specific disruption of the Brca1 gene display markedly impaired T-lymphocyte development and proliferation with increased chromosomal abnormalities. Here, BRCA1 is linked to thymus lymphoma.