Research has shown that extracts of this tree contain promising new antitumor agents that exhibit potent cytotoxicity—a toxic effect on cells that prevents their reproduction or growth—in vitro against human breast, colon and lung cancer cells. In fact, one particular substance from this tree was determined to have a potency over a billion times that of adriamycin—a standard anticancer drug. Extracts from this tree are derived from the twigs, which yield the most potent extract. What is the name of this important medicinal tree?