Germline TP53 gene mutations are associated with complex cancer predisposition syndrome, the Li-Fraumeni syndrome, and are not as rare as were previously thought. Currently, the identification of Li-Fraumeni syndrome is mostly based on a conformance to descriptive criteria, which recently were amended to include wider spectrum of malignancies. The presence of very young age-onset breast cancers in TP53 mutations families is a feature that overlaps with hereditary breast/ovarian cancer families with BRCA1/2 genes mutations. Peri-diagnostic germline TP53 testing results in breast cancer patients can significantly affect surgical and adjuvant radiotherapy choices. The aim of this case report is to emphasize the importance of peri-diagnostic germline TP53 molecular testing in patients with early-onset breast cancer and its effect on the management and outcome of the disease. We present the apparent BRCA1-related, although mutation negative, breast and ovarian cancer patient who subsequently was confirmed to be TP53 c.817C>T (p.R273C) mutation carrier and discuss the importance of peri-diagnostic oncogenetic TP53 testing in early breast cancer cases. Histopathology and genetic modifiers (MDM2 SNP309G; TP53 R72P, PIN3) data are also addressed.