Two studies in today's Nature Neuroscience, led by researchers at Boston Children's Hospital, Brigham and Women's Hospital (BWH), and Harvard Medical School (HMS), implicate mosaic mutations arising during embryonic development as a cause of autism spectrum disorder (ASD). The findings open new areas for exploring the genetics of ASD and could eventually inform diagnostic testing. Mosaic mutations affect only a portion of a person's cells. Rather than being inherited, they arise as a "mistake" introduced when a stem cell divides. A mutation in a stem cell will only be passed to the cells that descend from it, producing the mosaic pattern. When mosaic mutations occur during embryonic development, they can appear in the brain and affect the function of neurons. The earlier in development a mutation happens, the more cells will carry it.