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. Here, CYP19A1 is linked to cancer.