The overall goals are to change the paradigm for development of mechanism targeted pharmacotherapy in neurodevelopmental disorders and provide a definitive test of the mGluR theory in humans by determining whether AFQ056, an mGluR5 negative modulator, can enhance neural plasticity in the form of language learning during an intensive language intervention in very young children with fragile X syndrome. This trial therefore will use an innovative but exploratory new trial design to develop a different way to examine efficacy of an agent with substantial support as a drug targeting CNS plasticity in preclinical models of a developmental disorder. If the design is successful, this trial can serve as a model for future trials of mechanistically-targeted treatments operating on neural plasticity in other neurodevelopmental disorders.