High cholesterol is a risk factor for some breast cancers and is associated with reduced response to hormone therapy. 27-hydroxycholesterol (27HC) is a metabolite of cholesterol, and a recent study shows that it is specifically required for hormone-dependent growth and metastasis in a mouse model of breast cancer. Inhibiting cytochrome P450 oxidase (CYP27A1), the enzyme that converts cholesterol to 27HC, reduced tumour growth in these mice. Moreover, the authors showed that expression levels of CYP27A1 are correlated with tumour grade in humans. This molecule may represent a new therapeutic target for breast cancer treatment and prevention.