With its role in higher cognitive functions, the cortex represents a significant evolutionary development in mammals, culminating in the enlarged hemispheres of humans and other primates. In the development of this crowning structure, neurons are guided by factors that are both genetic and environmental. A research team led by Christopher A. Walsh, the Bullard professor of neurology at Beth Israel Deaconess Medical Center, recently discovered that one gene in particular is necessary to form the cortex, serving as a selective switch for development. The researchers reported the ironic finding that this gene, called Lhx2, is controlled by the roof plate, a brain structure to which researchers have not paid much attention. The existence of a gene that has such a powerful role in cortical development raises the question of whether it figures into human disease. Knowledge about Lhx2 may have implications for stem cell therapies and other methods of manipulating cells. “This is a gene that basically tells a neuron, you’re going to be part of the cortex,” said Walsh. “I think that’s an important concept to have a handle on if we want to replace lost neurons in the cerebral cortex.”