Is it ensure, insure, or assure?
ergosterol er·gos·ter·ol (ûr-gŏs'tə-rôl', -rōl')
A crystalline sterol synthesized by yeast from sugars or derived from ergot and converted to vitamin D2 when exposed to ultraviolet radiation.
a white crystalline organic solid of the molecular formula C28H44O belonging to the steroid family. It is found only in fungi (e.g, Saccharomyces and other yeasts and Claviceps purpurea, the cause of ergot, a fungal disease of cereal grasses) and is chemically related to cholesterol. Ergosterol is converted by ultraviolet irradiation into ergocalciferol, or vitamin D2, a nutritional factor that promotes proper bone development in humans and other mammals.