The brain consists of neurons and glial cells. Recent research uncovers the importance of glial cells; their developmental abnormality causes various diseases such as autism spectrum disorders and aberrant cerebral cortex development. Herein, CD38 gene knocking-out is shown to cause aberrant development of glial cells, especially, those called astrocytes and oligodendrocytes. As CD38 knockout mouse exhibits abnormal social behavior, these abnormal glial development may be one of the reason for the abnormal behavior. The present study suggests the importance of glial cells for the cerebral cortex development and social behavior.