27-Hydroxycholesterol (27-HC) is an endogenous oxysterol produced by the enzyme CYP27A1, with multiple biological functions, including acting as a selective estrogen receptor modulator (SERM) and as an agonist of the liver X receptor (LXR). 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.