Mixed RGNT and DNET are a rare brain tumor commonly found in the posterior fossa with the histologic features of both tumor types. Detailed genetic analyses of RGNT are rare and the most common mutation is the PIK3CA mutation of the PI3K/Akt/mTOR pathway. Documented genetic analyses of mixed RGNT/DNET are limited and have not tested for the PIK3CA mutation. This is the first documented case of the PIK3CA mutation in mixed RGNT/DNET; however it is possible that the PIK3CA mutation is common in this rare tumor subtype based on its prevalence in RGNT. Identification of the PIK3CA mutation in RGNT and RGNT/DNET may have therapeutic implications for chemotherapy targeting the PI3K/Akt/mTOR pathway.