For one set of experiments, mice were exposed to genistein for 4 days through their drinking water. Their oocytes were then collected and cultured in petri dishes. For the remaining experiments, the oocyctes were collected from animals that did not eat genistein and then exposed to 1, 5 or 10 micromoles (µM) of genistein in the petri dishes. In both cases, the oocyte collection and culturing procedure was similar to how in vitro fertilization is done in humans.
Blood levels in people eating soy-rich diets are generally in the range of 1-5 µM, so the exposure levels used in this study are similar to what a human fetus could be exposed to.