Medical researchers have been working toward implanting nerve precursor cells in the brains of patients with Parkinson’s and Huntington’s diseases. It was hoped that these cells would assume the function of the cells that have died off. However, the implanted nerve cells frequently did not migrate as hoped, rather they hardly move from the site. Scientists have now discovered an important cause of this: Attractants secreted by the precursor cells prevent the maturing nerve cells from migrating into the brain.