Folds in the cerebral cortex in mammals are believed to be indispensable for higher brain functions but the mechanisms underlying cortical folding remain unknown. By using the latest genome editing tools, we succeeded in establishing a technique and discovered important neurons for fold formation and the importance of the Cdk5 gene in those neurons. Some patients suffer from lissencephaly, whose cortical fold formation is impaired. Our study may provide clues to diseases including lissencephaly.