Scientists have made mice with a remarkable amount of room for human brain tissue to grow. By genetically engineering animals lacking part of their brain, researchers created a living model in which pea-sized clumps of human neurons could expand, mature and wire into a developing nervous system, ultimately coming to dominate the mouse cortex, the brain’s outer layer. The model offers a new way to study human neural circuits and brain disease, researchers report September 16 in Nature. “Our goal has been to make aspects of human brain development and function accessible for investigation, so we can develop therapeutics,” says...