Stanford researchers reported “xenocortical mice” engineered to make space for human neurons in vivo, creating an animal model with nearly half the cortex volume replaced by human cells. The approach builds on earlier work showing human brain organoids can survive in rodent brains, and it aims to improve modeling of brain development and neuropsychiatric disease biology. In the reported behavioral tests, mice with added human neural tissue performed better than genetically modified controls on a maze task, suggesting human tissue can contribute to aspects of cognition. The study also raises bioethics and translational questions around cross-species neural integration while offering a platform for injury and drug-testing research.