Breast cancers in women with coexisting thyroid cancer and CHEK2 mutation had tendency to express different histopathological and clinical features – tumors had more often positive estrogen and progesterone receptor status (especially in the group of breast cancer diagnosed below 51 years), were more often multifocal, none was bilateral nor had HER2 overexpression. This evidence concerns the gene CHEK2 and thyroid gland carcinoma.