Purity ≥95%. Rapamycin is a bacterial macrolide with antifungal and immunosuppressant activities (1). Rapamycin forms a complex with the immunophilin FKBP12 which then inhibits the activity of FRAP/mTOR (TOR in yeast) (2,3). Rapamycin treatment of cells leads to the dephosphorylation and inactivation of p70 S6 kinase. Rapamycin also leads to the dephosphorylation of 4E-BP1/PHAS1, thereby promoting its binding to and inactivation of eIF4E (4,5). This activity has been shown to be the basis for Rapamycin's ability to block protein synthesis and to arrest cell cycle progression in the G1-phase (6,7). However, it has been suggested that Rapamycin's inhibition of the G1/S transition may be the consequence of its effect on cyclin D1 mRNA and protein stability (8). Synonyms: Sirolimus; Rapamune; NSC 226080; Antibiotic AY-22989; WY-090217; RAPA; SILA 9268A; CCRIS 9024; mTOR Inhibitor; 23,27-Epoxy-3H-pyrido[2,1-c][1,4]oxaazacyclohentriacontine