The block of both repair pathways promotes the apoptosis of tumor cells, which is called the “synthetic lethal” mechanism of PARP inhibitors, while for breast cancer patient without pathogenic BRCA1/2 mutations, HRD score can be predicted with gene expression of five DNA helicase genes using the linear regression model to further select breast cancer patients with HRD phenotype. The gene discussed is PARP1; the disease is breast cancer.