In an extraordinary new paper, just out, in Nature @Sergiu_P_Pasca and his team genetically altered mice to make them "apallial" (missing a cortex). And then, postnatally, transferred organoids derived from human neural cells into that space. Three months later, in the animals measured, the cortex was 91.9% human-derived neural cells by volume. The result is not quite a mouse mouse (albeit I would still say, it's a mouse!), and certainly not anything like a human. Nothing the researchers measured suggests these animals have human-like experiences. What a model like this promises for neurological disease modeling and drug discovery cannot be overstated. It's nothing short of an extraordinary breakthrough. It also raises complex ethical questions (to be clear, these researchers carefully engaged external ethicists (I was one of them), and followed every ethical and legal guideline possible.) Which is why In a forthcoming commentary, Anil Seth and I explore what an ethical roadmap forward would be for this work. We address whether these mice chimeras could have any human-like capacities, how we might measure and know, and who, and how, we decide together what the next steps in this research should be.