Over the next several years, Levi-Montalcini focused on searching for the mysterious trophic factor that she had intuited during the war. A former student of Hamburger’s had fortuitously noticed that a certain mouse tumor cell line— called sarcoma 180—caused more nerve cells to grow. When Levi-Montalcini incorporated the tumor cells into developing chicks, she observed the same effect. Something in the tumor caused the differentiation of the nerve cells to accelerate; it also caused the creation of excessive numbers of nerve fibers.
Levi-Montalcini started trying to isolate the trophic factor and began to collaborate with biochemist Stanley Cohen, then at Washington and now at the Vanderbilt University School of Medicine. They found that the partially purified factor contained both protein and nucleic acid. By adding enzymes from snake venom—which breaks down these compounds—in hopes of determining which component contained the biological activity, the two discovered that the venom itself contained the factor.