Genistein is a molecule that has been shown in studies to be toxic to a variety of cancer cells. It's also found in soy, specifically in tofu, soy flour, soy protein isolates and some dietary supplements.
When taken in conjunction with chemotherapy, soy isoflavones such as genistein have shown to make chemo more efficient and work faster by helping the drugs kill tumor cells or stop those cells from dividing. In breast cancer, for example, genistein has been shown to reduce tumors by blocking the cancer-causing effect of estrogen, but in other studies has shown to encourage estrogen-dependent breast tumors -- about 70 percent of women with breast cancer have estrogen-responsive tumors.
Important: Talk to Your Doc
Depending on what type of cancer you're being treated for, it's best to discuss adding soy to your diet with your physician beforehand, as there are contradictory findings regarding its efficacy.