AbstractObjectiveLi-Fraumeni syndrome (LFS) is a pan-cancer predisposition syndrome caused by germline pathogenic variants in the geneTP53. The interpretation ofTP53variants in clinical scenarios outside the classic LFS criteria may be challenging. Here, we report a patient affected by 2 primary cancers at later ages, who harbored a likely pathogenicTP53at low allele frequency detected in a blood sample.MethodsThe Molecular Tumor Board committee at our institution revisited the case of a patient who was enrolled in a research protocol for the investigation of genetic conditions associated with neuroendocrine tumors. Clinical, familial, and molecular data were reviewed. The patient received germline testing using a next generation sequencing multi-gene panel and was incidentally found to harbor aTP53likely pathogenic variant, with 22% of variant allele fraction. Additional samples, including a second blood sample, oral swab, and saliva, were collected for DNA analysis. A newTP53sequencing round was performed with the attempt to distinguish between a true constitutional germline variant and a somatically acquired variant due to aberrant clonal expansion of bone marrow precursors.ResultsPatient’s personal and familial history of cancer did not meet classic nor Chompret LFS criteria. Environmental risk factors for cancer were identified, such as alcohol abuse and tobacco exposure. The TP53 variant initially found in the next-generation sequencing was confirmed by Sanger sequencing in the previous DNA sample extracted from blood for the first analysis and in a second blood sample collected 6 years later. The TP53 variant was not detected in the DNA extracted from the oral swab and saliva samples.ConclusionConsidering the low TP53 variant allele fraction in blood, absence of variant detection in oral swab and saliva samples, the lack of LFS clinical criteria, and history of exposure to environmental risk factors for cancer, the main hypothesis for this case was aberrant clonal expansion due to clonal hematopoiesis. Oncologists should interpretTP53findings during germline testing with caution.