In the first step of our analysis, we focused on the canonical genes predisposed to HBOC, such as ATM, PALB2, Chek2, CDH1, PTEN, and TP53, but no variants in these genes were detected except for two missenses in the ATM gene, c.1810C>T; p.Pro604Ser and c.6115G>A; p.Glu2039Lys, which were identified in two patients from the BC-F15 and BC-F16 families diagnosed with stage-II BC at the ages of 30 and 27 years, respectively. Here, CHEK2 is linked to breast cancer.