Autism is a group of developmental brain disorders, collectively called autism spectrum disorder (ASD). The term "spectrum" refers to the wide range of symptoms, skills, and levels of impairment, or disability, that children with ASD can have. Some children are mildly impaired by their symptoms, but others are severely disabled. ASD is diagnosed according to guidelines listed in the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition - Text Revision (DSM-IV-TR). The manual currently defines five disorders, sometimes called pervasive developmental disorders (PDDs), as ASD: Autistic disorder (classic autism) Asperger's disorder (Asperger syndrome) Pervasive developmental disorder not otherwise specified (PDD-NOS) Rett's disorder (Rett syndrome) Childhood disintegrative disorder (CDD).