Fragile X syndrome (FXS) is the most common form of inherited intellectual disability and one of the main identified genetic causes of autism spectrum disorder. Our understanding of neuropathophysiology of FXS is mainly gained from studies in the fmr1 null mouse model. However, these studies don’t always translate to the human condition due to species and possibly the mode of gene silencing in FXS. These limitations have been recently circumvented by the advent of human induced pluripotent stem cells (hiPSCs), and the generation of FXS patient-derived hiPSCs, which presents an opportunity for studying the pathogenesis of FXS with unlimited human brain cells. Although astrocytes, non-neuronal cells in the brain, have extensive roles in normal brain function, are increasingly implicated in multiple brain disorders and express FMRP into adulthood, a comprehensive understanding of the role of astrocytic FMRP in the pathogenesis of FXS is still lacking. We will therefore develop a human cellular model of FXS by differentiating astrocytes from control and FXS induced hiPSCs. In order to determine how FXS astrocytes are impaired and how they impact formation and function of neural circuits, we propose to create a humanized chimeric mouse model with hiPSCs. In Aim1 we will examine how FXS astrocyte morphology, Ca2+ signaling and gene expression are impaired in vivo. In Aim 2, we will determine the effect of FXS hIPSC-astrocytes on neuronal structure and function. This work is expected to provide important knowledge about the role of human specific astrocytes in FXS and has the potential to identify novel therapeutic targets for FXS.