A sterol found in yeasts and fungi and in plant and animal fat. Under the action of sunlight or ultraviolet rays it produces vitamin D2. The substance produced in this way is known as calciferol, and is used for the prevention and cure of RICKETS and OSTEOMALACIA. A similar change in the ergosterol of the skin is produced when the body is freely exposed to sunlight. Calciferol is probably not so active as, and differs chemically from, the vitamin D occurring in fish-liver oils. (See APPENDIX 5: VITAMINS.)