Overall, BC is a heterogeneous and polygenic disease [38], with 10–15% of BC cases being caused by hereditary/germline mutations in BC susceptibility genes [39], known as high-penetrance alleles/high-risk variants (i.e., BRCA1, BRCA2, TP53, STK11, CD1 and PTEN), moderate-penetrance alleles/moderate-risk variants (i.e., ATM, PALB2, CHEK2, BRIP1, RAD51C) and common low-penetrance alleles/low-risk variants [40]. This evidence concerns the gene CHEK2 and breast cancer.