Spontaneously arising de novo mutations are likely important contributors to the development of autism spectrum disorder (ASD) in affected individuals. Genetic mutation is an ongoing process. Although people inherit deleterious mutations from their parents, some can also arise postzygotically. Freed and Pevsner examined the degree to which postzygotic mutations contribute to ASD and found mosaic mutations, which occur postzygotically and only in a subset of a person’s cells, in multiple genes implicated in ASD. On the basis of these observations, the authors estimated that mosaic mutations contribute to ∼5% of ASD diagnoses.