Professor Carla Shatz, Professor of Biology and of Neurobiology and the Director of Bio-X at Stanford University, discusses research results that show that Major Histocompatability Class I and PirB genes, thought previously to function only in immunity, act at neuronal synapses to limit how much - or how quickly - synapse strength changes in response to new experience. Changes in their function could contribute to developmental disorders such as Schizophrenia, and even to the synapse loss in Alzheimer’s Disease.