A cyclic hydrocarbon alcohol commonly classified as a lipid because it is insoluble in water but soluble in a number of organic solvents. It is the major sterol in vertebrate cells and the most common sterol of eukaryotes, but it is absent from most prokaryotes. In vertebrates, the highest concentration of cholesterol is in the myelin sheath that surrounds nerves and in the plasma membrane that surrounds all cells. Insects and most invertebrates cannot synthesize cholesterol; therefore, they rely on dietary cholesterol for use in their membranes and as a precursor of ecdysone, a molting hormone required during growth and development.