Cholesterol, a sterol, is a type of lipid biosynthesized by all animal cells. It is an essential structural component of animal cell membranes. It is also the precursor for vitamin, bile acid and steroid hormones in mammals. Cholesterol can complex with proteins to form two main types of lipoprotein, high-density lipoprotein (HDL) and low-density lipoprotein (LDL). Cholesterol is associated with breast cancer progression.