During her research at Washington University in St. Louis, Missouri, she discovered nerve growth factor, or NGF, the first substance known to regulate the growth of cells. She showed that when tumors from mice were transplanted to chicken embryos they induced rapid growth of the embryonic nervous system. She concluded that the tumor released a nerve growth-promoting factor that affected certain types of cells.
The research increased the understanding of many conditions, including tumors, developmental malformations, and senile dementia. It also led to the discovery by Stanley Cohen of another substance, epidermal growth factor, which stimulates the proliferation of epithelial cells.
Wikipedia: Rita Levi-Montalcini