Breast cancer (BC) is not a typical manifestation of Lynch syndrome. The existence and extent of excessive breast cancer risk in carriers of pathogenic mutations in the Lynch syndrome-associated genes (MLH1, MSH2, MSH6, PMS2) remains an open question. In addition, it is known that some of the breast neoplasms in patients with this syndrome are causally linked to the hereditary mutation, and some arise completely independently of the hereditary defect in the gene of the DNA mismatch repair system. In the case of accidental detection of such germline mutations in breast cancer patients, a thorough differential diagnosis between these categories of tumors is required, and the result of it is actionable, requiring changes in the management. This is a report of a case of breast cancer that arose in a carrier of a pathogenic mutation in the PMS2 gene, which was an accidental finding. The description of molecular genetic diagnostics is given: the microsatellite markers assessment and the detection of «loss of heterozygosity» allowed to classify the neoplasm in a category of cases that developed without any causal link to the patient's Lynch syndrome.