The team behind a Nature study reports a model it calls “xenocortical mice” (XCX) for studying human brain development and disease. It says preventing about 98% of the mice’s cortex and hippocampus from forming creates space for transplanted human cortical organoids—lab-grown models of brain tissue—to grow and integrate extensively. A post sharing the announcement calls the work promising for translational research, but argues that human–mouse brain chimeras raise welfare questions and strengthen the case for a science of welfare encompassing both biological and artificial systems.