For example, in cancer cells with mutations in BRCA1 or BRCA2, which are involved in DNA double-strand breaks, the mutations by themselves are not detrimental to the survival of the cancer cells (rather, the accumulation of genetic mutations is likely to cause clonal evolution), but the administration of a DNA single-strand break repair enzyme inhibitor, PARP inhibitor, results in death. The gene discussed is BRCA1; the disease is cancer.