Mark Bear, Picower Professor of Neuroscience at MIT, recalls the “eureka moment” 20 years ago when he realized that a severe developmental brain disorder—fragile X syndrome—might be treated with drugs that inhibit a neurotransmitter receptor called mGluR5. The idea, that mGluR5 stimulates excessive protein synthesis in fragile X neurons that disrupts their functions, became well validated by experiments in his lab and others worldwide using several animal models of the disease.