The gene knockout method using ES cells has played a major role in investigating the function of genes. Particularly in behavioral research, it is an effective method in that it can directly show how the mutation of genes affects the ultimate behavior. However, there is a problem that simple knockouts in which genes are deleted in any tissue from the time of fertilized eggs do not know when and where gene function was important. In this lecture, discussions will be made based on specific research examples on how to approach such problems using genetic engineering methods.