The diagnostic category pervasive developmental disorders (PDD), as opposed to specific developmental disorders (SDD), refers to a group of five disorders characterized by delays in the development of multiple basic functions including socialization and communication. The pervasive developmental disorders are pervasive developmental disorder not otherwise specified (PDD-NOS), which includes atypical autism and is the most common; autism, the best-known, now understood to be part of a spectrum; Asperger syndrome; Rett syndrome; and childhood disintegrative disorder (CDD).