Human induced pluripotent stem cell (iPSC)-derived neurons could be used to model CNS development. Real et al. transplanted cultures of green fluorescent protein-expressing neurons and neural progenitors derived from human iPSCs into the somatosensory cortex of adult mice, and used multiphoton imaging to longitudinally track the grafts over 4 months. Grafts showed changes similar to those in developing human cortex, including axon growth, synapse formation and turnover, and oscillatory activity. By contrast, grafts derived from individuals with Down syndrome showed a lower synapse turnover rate, indicating that iPSC-derived cells can also be used to model aspects of neurodevelopmental disorders.