Grifola frondosa is a polypore mushroom that grows in clusters at the base of trees, particularly oaks. The mushroom is commonly known among English speakers as hen of the woods, hen-of-the-woods, ram's head and sheep's head. It is typically found in late summer to early autumn. In the United States' supplement market, as well as in Asian grocery stores, the mushroom is known by its Japanese name maitake (舞茸, "dancing mushroom"). Throughout Italian American communities in the northeastern United States, it is commonly known as the signorina mushroom. G. frondosa should not be confused with Laetiporus sulphureus, another edible bracket fungus that is commonly called chicken of the woods or "sulphur shelf". Like all polypores, the fungus becomes inedible when older, because it is then too tough to eat.
The fungus is native to China, the northeastern part of Japan and North America, and is prized in traditional Chinese and Japanese herbology as a medicinal mushroom.
In 2009, a phase I/II human trial, conducted by Memorial Sloan–Kettering Cancer Center, showed maitake could stimulate the immune systems of breast cancer patients. Small experiments with human cancer patients have shown it can stimulate immune system cells, such as NK cells In vitro research has also shown Maitake can stimulate immune system cells. An in vivo experiment showed that it could stimulate both the innate immune system and adaptive immune system.
In vitro research has shown maitake can induce apoptosis in various cancer cell lines, and inhibit the growth of various types of cancer cells. Small studies with human cancer patients revealed that a portion of this mushroom, known as the mitake D-fraction, possesses anticancer activity. In vitro research demonstrated the mushroom has potential antimetastatic properties.
Maitake has a hypoglycemic effect, and may be beneficial for the management of diabetes. It lowers blood sugar because the mushroom naturally contains an alpha glucosidase inhibitor.
This species contains antioxidants and may partially inhibit the enzyme cyclooxygenase. An extract of maitake inhibited angiogenesis via inhibition of the vascular endothelial growth factor.
Lys-N is a unique protease found in maitake. Lys-N is used for proteomics experiments because of its protein cleavage specificity.