Neurons often get most of the credit for keeping our brains sharp and functioning—as well as most of the blame when it comes to brain diseases. But star-shaped cells called astrocytes, another abundant cell in the human brain, may bear the brunt of the responsibility for exacerbating the symptoms of some neurodevelopmental disorders. Salk Institute scientists have now identified a molecule produced by astrocytes that interferes with normal neuron development in Rett, fragile X and Down syndromes.