Because of the critical role of the mTOR signaling pathway in tumor cell growth and relevant anti-cancer activities, specific rapamycin analogues or derivatives have been evaluated by the FDA for treating renal cell carcinoma.[2] A phase III clinical trial study was also conducted on breast cancer patients, in which sirolimus was tested in combination with letrozole, an aromatase inhibitor. This evidence concerns the gene CYP19A1 and hereditary clear cell renal cell carcinoma.