Given that homologous recombination (HR) deficiency is commonly associated with BRCA1 loss and basal-like breast cancer, we hypothesized that ER-positive breast cancer—where BRCA1 is typically intact—would provide a valuable model to study the effect of EXO1 overexpression on HR (Lord and Ashworth, 2016; Feng et al., 2023). This evidence concerns the gene BRCA1 and breast cancer.