A study of over 2,300 nerve cells has revealed that the developing human brain eliminates mutant neurons before birth. In fetal brains, more than a quarter of neurons exhibit copy number variation abnormalities involving gains or losses of chromosome segments, whereas in newborn brains this proportion is only about 6%, indicating that a large number of abnormal neurons are cleared during development. The researchers speculate that brain cells with high levels of copy number variation often show abnormal activity in synapse-related genes, and the brain may use this as a mechanism to identify and eliminate neurons with abnormal connections. This process regulates both the number of neurons and the quality of those retained; if it malfunctions, it may trigger neurological disorders such as autism spectrum disorder and epilepsy, offering a new direction for understanding these diseases.