(25R)-cholest-5-ene-3β,26-diol

27-Hydroxycholesterol (27-HC) is an endogenous oxysterol with multiple biological functions, including activity as a selective estrogen receptor modulator (SERM) (a mixed, tissue-specific agonist-antagonist of the estrogen receptor (ER)) and as an agonist of the liver X receptor (LXR). It is a metabolite of cholesterol that is produced by the enzyme CYP27A1.
A link between high cholesterol and breast cancer has been identified, and it has been proposed that this is due to 27-HC production by CYP27A1. Because of its estrogenic action, 27-HC stimulates the growth of ER-positive breast cancer cells, and has been implicated in limiting the effectiveness of aromatase inhibitors in the treatment of breast cancer. As such, identified CYP27A1 inhibitors, including the marketed drugs anastrozole, fadrozole, bicalutamide, dexmedetomidine, ravuconazole, and posaconazole, have been proposed as potential adjuvant therapies in ER-positive breast cancer.