Thanatophoric dysplasia (TD) is an intractable disease with abnormalities of bones and the brain. Because of experimental difficulties, its pathophysiology is largely unknown. In this study, by combining ferrets, whose brain is rather similar to that of humans, and unique technique developed by the present researchers, neuronal translocation along radial glial fibers to the cerebral cortex during fetal brain development has been discovered to be aberrant, which suggests the cause underlying TD.