Background: Multi-gene hereditary cancer panels have revolutionized how patients with germline mutations are identified by testing multiple genes at the same time. Despite the availability of panel testing, many patients with a known familial mutation will only undergo single site genetic testing due to limitations in guideline recommendations and insurance coverage. This approach risks a failure to detect additional pathogenic variants and an inappropriate management of cancer risk. In our clinical experience, a subset of patients pursue multigene testing despite a known familial mutation. Our group has identified patients who carry more than one mutation and mutations that would have been missed if the patients had only undergone single site testing. We investigated the patients and families from our risk assessment clinic with multiple familial mutations and determined how medical management may have been changed due to the presence of multiple mutations in family. Methods: The Fox Chase Cancer Center Risk Assessment Program (RAP) Registry was queried to identify patients who carry more than one mutation. Pedigrees of patients and families identified with multiple germline mutations were reviewed. Screening management guidelines were determined from the most recent NCCN guidelines published at the time the patient tested (Genetic/Familial High-Risk Assessment: Breast, Ovarian, and Pancreatic and Genetic/Familial High-Risk Assessment: Colorectal). The RAP Registry is an IRB approved protocol (IRB 09-831). Results: 70 patients were found to carry at least 2 mutations (excluding patients with biallelic MUTYH mutations) since introducing multi-gene panel testing in 2014. The most common second mutation was the I1307K variant in the APC gene at 20% (14/70). We also identified 20 patients who would have received incomplete genetic risk assessment if they only underwent single site testing and screening management changed in 60% (12/20) of these patients. 35% (7/20) of these patients did not meet NCCN criteria for additional germline testing beyond single site testing. Conclusions: Multi-gene hereditary cancer panels identify patients and families with multiple germline mutations. Patients undergoing single site cascade testing are at risk of receiving inaccurate risk assessment based on incomplete ascertainment of germline cancer risks. Detection of additional actionable mutations will frequently lead to changes in medical management.