Neuroscientists in the US have successfully adapted mice to have functioning human cells inside their own brains.
The researchers hope that potential treatments for psychiatric and neurodevelopmental diseases that only occur in humans could now be tested on the laboratory rodents.
While mice with brains that are partly human might sound like a Kafkaesque experiment, the scientists said these are not "mice that think like humans".
The animals are genetically engineered - and surgically altered - so that some of their brain tissue is human.
The scientists said their work, published in the journal Nature, external, was carried out with independent ethical scrutiny.
The aim of this ethically complicated breakthrough was to better understand the biology of brain disorders for which there are now no effective treatments.
As lead researcher Prof Sergiu Pașca from Stanford University explained in a press conference, psychiatry has "one of the lowest success rates for clinical trials".
The Stanford researchers said that, for some conditions, including epilepsy, autism and cerebral palsy, it has the potential to be "transformative".
The human brain is made up of billions of cells, interconnected in millions of circuits.
After a few months though, the human cells started to look and function like the outer layer of the mouse's brain.
Dr Sarah Chan, a reader in bioethics at the University of Edinburgh who was not involved in this research, told BBC News there was "no indication that what's being created here are mice that can think like humans, or a human brain in a mouse body."