An isoflavonoid derived from soy products. It inhibits protein-tyrosine kinase and topoisomerase-II (DNA topoisomerases, type II) activity and is used as an antineoplastic and antitumor agent. Experimentally, it has been shown to induce G2 phase arrest in human and murine cell lines.
Additionally, genistein has antihelmintic activity. It has been determined to be the active ingredient in Felmingia vestita, which is a plant traditionally used against worms. It has also been demonstrated to be effective against intestinal parasites such as the common liver fluke, pork trematode and poultry cestode. [Wikipedia]
Further, genistein is a phytoestrogen which has selective estrogen receptor modulator properties. It has been investigated in clinical trials as an alternative to classical hormone therapy to help prevent cardiovascular disease in postmenopausal women. 
Genistein can be found in food sources such as tofu, fava beans, soybeans, kudzu, and lupin. It is also present in certain cell cultures and medicinal plants. [Wikipedia]
Humans and other mammals
Parasitic nematodes and other roundworms
Currently Genistein is being studied in clinical trials as a treatment for prostate cancer.
Mechanism of action
Genistein may inhibit cancer cell growth by blocking enzymes required for cell growth.
Genistein may decrease cardiovascular risk in postmenopausal women by interacting with the nuclear estrogen receptors to alter the transcription of cell specific genes. In randomized clinical trials, genistein was seen to increase the ratio of nitric oxide to endothelin and improved flow-mediated endothelium dependent vasodilation in healthy postmenopausal women.  In addition, genistein may have beneficial effects on glucose metabolism by inhibiting islet tyrosine kinase activity as well as insulin release dependent on glucose and sulfonylurea.