XRCC2 and breast cancer: Such genes exhibit variable penetrance, and the well-known ones include high-penetrance genes (BRCA1, BRCA2, TP53, PTEN, STK11 and CDH1) and moderate-penetrance ones (STK11/LKB1, PALB2, CHECK2, ATM, MRE11, RAD50, EAD51, RAD51B, RAD51C, RAD51D, NBS1, BRIP1, FANCA, FANCC, FANCM, and XRCC2), which account for approximately 3-6% and 5-10% of breast cancers, respectively (67).