BRCA1 and breast cancer: Up to 25% of BC are associated with highly penetrant genes such as BRCA1, BRCA2, TP53, CDH1, PTEN, and STK11. Another 2-3% of cases result from rare, moderate-penetrance gene mutations like those in CHEK2, BRIP1, ATM, and PALB2, each doubling the risk [15, 16].