Fertility experts have long been interested in understanding why so few human eggs harvested during in vitro fertilization result in pregnancies. Researchers found that eggs that matured in culture, as opposed to eggs that were already mature at the time of harvesting, tended to lack gene activity normally involved in the development of the cytoplasm of the egg, or the area outside the nucleus. Furthermore, they compiled a list of mRNA-mediated gene expression changes that take place as the eggs matured.