In our retrospective analysis of transcriptome data from BRCA1-deficient ovarian cancer patients, we found that the DNA damage repair-related pathways were severely attenuated, and the gene mutation frequency was much higher than that in patients with normal BRCA1, suggesting that BRCA1 deficiency caused inefficient DNA damage repair, leading to a massive number of gene mutations and extreme genome instability. Here, BRCA1 is linked to ovarian cancer.