A team of researchers from Stanford University in the United States has created mice whose cerebral cortex — the part of the brain responsible for complex functions like thought and memory — is largely composed of human cells. The findings, published in the journal Nature on September 16, 2026, mark a significant step in neuroscience and raise new ethical questions. The study, led by neuroscientist Sergiu P. Pașca, aimed to better understand human brain development and diseases by using a living model, as human brain tissue is difficult to study directly and traditional animal models do not fully replicate human biology. To achieve this, the researchers genetically modified the mice so that their own cerebral cortex and hippocampus — a brain region important for memory — did not develop properly. This allowed human cortical tissue, grown in the lab, to integrate into the mouse brain. Once transplanted, the human cells formed connections with the mouse nervous system, sending signals and creating circuits. However, the human cells continued to develop at their own pace, slower than the mouse's, and after several months, they were still at a stage similar to the middle of human pregnancy. The human brain cells in the mice generated a wide variety of cell types, including von Economo neurons, which are rare and associated with complex social behaviors in humans and some other mammals. These neurons had not previously been observed in similar studies. The research also noted that human brain tissue was more vulnerable to oxygen deprivation than mouse tissue, highlighting potential limitations in using such models for certain studies. Despite these findings, the mice behaved normally in most respects, showing no signs of human-like intelligence or consciousness. Researchers caution against interpreting small behavioral differences too broadly and emphasize that the mice still have a mouse nervous system, not a human one. The team warns against using terms like "humanized mice" or "mice with a human brain," which could be misleading. Future research will focus on using this model to study neurological and psychiatric disorders by introducing genetic changes linked to these conditions into the human cortical tissue and observing how they affect development and brain function.