Bioprospecting is the search for new pharmaceutical, nutritional, or agricultural products from natural sources, including plants, animals, or microorganisms. Most commonly, it refers to the search for new medicines from plants or microbes. Plants have always been an important source of medicines, and approximately 25% of prescription pharmaceuticals contain a plant-derived ingredient. Examples of plant-derived drugs include the antihypertensive reserpine from Rauvolfia serpentina; vincristine and vinblastine, used to treat childhood leukemia and Hodgkin's disease, and derived from Catharanthus roseus; and taxol, derived from Taxus brevifolia and used to treat ovarian cancer.