CHEK2 and breast cancer: Pathogenic/likely pathogenic variants in the BRCA1 gene were found more often in patients with ER-negative BC compared to ER-positive BC (10.9% vs. 4.2%, p < 0.001), while P/LP variants in CHEK2 were exclusively identified in BC patients with ER-positive breast cancer (3.4% vs. 0%, p < 0.001) (Table S4, Figure 5a).