As of today less than 5% of familial breast cancer were attributed to high penetrance breast cancer genes BRCA1, BRCA2, PTEN, MSH2, STK11, CDH1, and TP53 (Wooster and Weber, 2003; Malone et al., 2006; Walsh et al., 2006) and rare genetic variants at ATM, CHEK2, BRIP, NBN, RAD50, or PALB2 that jointly confer an approximately two-fold increased risk (Meijers-Heijboer et al., 2002; The CHEK2 Breast Cancer Case-Control Consortium, 2004; Rahman et al., 2007). Here, ATM is linked to breast cancer.