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Cholesterol molecule, illustration. Cholesterol is an important component of the membranes of cells. It is the major precursor for a variety of steroid hormones, is involved in the production of vitamin D and has an important role for brain synapses and in the immune system. Overproduction leads to fatty deposits in blood vessels which can lead to heart disease, stroke, arterial disease, diabetes and high blood pressure. (KEYSTONE/SCIENCE PHOTO LIBRARY/KATERYNA KON/SCIENCE PHOTO LIBRARY)