Through the genome sequence analysis of mock-treated and MMS-treated chicken DT40 cell clones we show that BRCA1 or BRCA2 deficiency strongly and indistinguishably increases the level of spontaneously arising base changes with a mutation spectrum very similar to that observed in BRCA1/2-deficient cancer, and also increases mutagenesis induced by high doses of mutagens. The gene discussed is BRCA1; the disease is cancer.