e13101 Background: Germline TP53 mutations are associated with Li-Fraumeni syndrome (LFS), a disease that predisposes carriers to a wide variety of early onset tumors. Pre-menopausal breast cancer is the most frequent tumor found in carriers. In Southern and Southeastern Brazil, a germline TP53 mutation in exon 10, p.R337H, was diagnosed in 0,3% of the population due to a founder effect, a prevalence that is substantially higher than other LFS mutations. Methods: In this study we investigated the prevalence of the p.R337H TP53 germline mutation in 287 young female breast cancer patients (diagnosed before the age of 40) at A. C. Camargo Cancer Center, Sao Paulo, Brazil. DNA was extracted from saliva and Sanger sequencing of exon 10 was performed. All patients were consented. Results: The p.R337H TP53 mutation was present in 1.74% (5/287). Family history of cancer was present in three cases although it did not fulfill LFS clinical criteria. Conclusions: The identification of germline p.R337H mutation in this cohort strengthen the role of TP53 mutations in the early onset of breast cancer in pre-menopausal cancer in Brazil.