A large number of substances form liquid crystals, many of which are biological in nature. In fact, the very first liquid crystal to be heavily researched was myelin, a soft, fatty substance that sheathes certain nerve fibers and axons. The tobacco mosaic virus, which is one of the most studied helical rod-shaped viruses and is spread mechanically via abrasive contact with infected sap, passes through a nematic liquid crystalline phase. Cholesterol similarly forms a nematic phase, but a more organized one distinguished by a repeated rotation of the direction in which the component molecules are aligned. This unique phase is called the cholesteric phase and its characteristic rotation is caused by an inherent twist present in molecules of cholesterol.