Given this uncertainty, our aim was to assess, within a large international cohort, potential differential associations of treatment given for the first primary BC (i.e., radiotherapy, chemotherapy, and endocrine therapy) by CHEK2 c.1100delC status with CBC risk and to investigate whether the worse breast cancer‐specific survival (BCSS) so far reported in carriers is explained solely by the increased CBC risk. Here, CHEK2 is linked to breast cancer.