FBXW7 is a tumor suppressor gene that is mutated in several tumors including colorectal, liver, bladder and ovarian cancers. It is also mutated in endometrial, esophageal, and head and neck squamous cancers. FBXW7 is a tumor suppressor gene responsible for the degradation of several proto-oncogenes. mTOR is one of the substrates of FBXW7-mediated protein degradation, and loss of function of FBXW7 increases the levels of total and activated mTOR. FBXW7 R505G lies within the WD repeat 4 of the FBXW7 protein. R505G has been identified in the scientific literature, but has not been biochemically characterized and therefore, its effect on FBXW7 protein function is unknown. Preclinical data suggest that FBXW7 mutations may sensitize cells to mTOR inhibitors, however, the response to mTOR inhibitors and the clinicopathologic effects of FBXW7 R505G remains to be further elucidated.