We have recently showed that transplanting stem cell-derived organoids into the brains of rodents allows for the maturation of these cells into the early postnatal stage of human brain development, a time in which the pathogenesis of neurodevelopmental disorders is thought to occur. These transplanted human cortical neurons not only mature and replicate disease phenotypes with greater sensitivity compared to organoids grown in vitro, but also connect with host circuits, which can be used to model human diseases associated with behavioral deficits.