Ergosterol (ergosta-5,7,22-trien-3β-ol) is a sterol found in cell membranes of fungi and protozoa, serving many of the same functions that cholesterol serves in animal cells. Because many fungi and protozoa cannot survive without ergosterol, the enzymes that create it have become important targets for drug discovery. Ergosterol is a provitamin form of vitamin D₂; exposure to ultraviolet (UV) light causes a chemical reaction that produces vitamin D₂.