Phospholipids are important in the structure of all membranes. Their structure is similar to that of triglycerides, but they contain only two polyunsaturated fatty acids. The third molecule attached to the glycerol is a phosphatidylcholine molecule (choline is one of the B vitamins). The most common phospholipid is lecithin. Certain phospholipids also contain inositol (B vitamin) and designated as phosphatidylinositol. Phosphatidylethanolamine is another phospholipid that has several functions, such as being a precursor to choline and acetylcholine. Lecithin is found in highest concentration in soybeans and egg yolk. Recently, egg lecithin has been used in the treatment of Acquired Immune Deficiency Syndrome (AIDS).