There are seven species of Yew trees growing around the world, mainly in the Northern Hemisphere, and several ornamental hybrids. All are known to be very toxic and contain poisonous alkaloids except, due to an idiosyncrasy of nature, Pacific Yew (Taxus brevifolia) which is the only special safe to use as a dietary supplement.
The only plants on earth that produce naturally occurring compounds called "Texanes" are of the genus Taxus. Texans have been shown through scientific biological studies to inhibit harmful cell development and reproduction. The largest concentrations of wild Yews left on earth grow in the Pacific Northwest of the United States and are especially prolific in Northwest Montana. They are not threatened or endangered.
Centuries old use of Yew as a medicinal herb by Native Americans is documented with some tribes called it, "Chief of the Forest". Some uses were to gain strength and endurance and as a tonic for old people.