Here, we comprehensively and simultaneously evaluated the BRCA1/53BP1/PARP-1 repair network in three groups (HER2-positive, grade II-III hormone receptor [HR]-positive/HER2-negative and TN) of sporadic breast cancers (n = 155) from patients without familial breast cancer history or known BRCA1 mutations to identify tumour population(s) with a theoretically high susceptibility to PARPi. The gene discussed is BRCA1; the disease is breast cancer.