Maitake is an edible mushroom from the species Grifola frondosa. Maitake D-fraction® is an extract of this large mushroom native to the mountains of northeastern Japan. The maitake mushroom is eaten as a food, and maitake-D fraction is marketed as a dietary supplement in the United States and Japan. The substance in the maitake mushroom is thought to be active in humans and is called beta-glucan. Research has shown that maitake D-fraction has effects on the immune system in animal and laboratory studies. There is no convincing clinical evidence to date in available peer-reviewed medical journals reporting that the maitake mushroom is effective in treating or preventing cancer in humans, although some human research is now underway.