27-Hydroxycholesterol is an endogenous selective estrogen receptor modulator that displays significant partial agonist activity in a variety of cellular models of estrogen receptor action. It positively regulates both gene transcription and cell proliferation in cellular models of breast cancer. 27-Hydroxycholesterol, through estrogen receptor activation, triggers deleterious effect in prostate cancer cell lines. 27-Hydroxycholesterol significantly increases cell proliferation of LNCaP and PC3 cells and this effect can be attenuated by estrogen receptor inhibitors. 27-Hydroxycholesterol is an oxysterol produced from cholesterol by the monooxygenase CYP27A1, which regulates intracellular cholesterol homeostasis. 27-Hydroxycholesterol also acts as an endogenous selective estrogen receptor modulator capable of increasing breast cancer growth and metastasis. 27-Hydroxycholesterol levels can be modulated by statins or direct inhibition of CYP27A1, thereby attenuating its pro-tumorigenic activities. 27-hydroxylation of cholesterol is an important pathway for LXR activation in response to cholesterol overload.