Autism Spectrum Disorders, sometimes called Pervasive Developmental Disorders (PDD), are a range of neurological disorders that most markedly involve some degree of difficulty with communication and interpersonal relationships, as well as obsessions and repetitive behaviors. As the term "spectrum" indicates, there can be a wide range of effects. Those at the lower-functioning end of the spectrum may be profoundly unable to break out of their own world and may be described as having Kanner's autism. Those at the higher-functioning end, sometimes diagnosed with Asperger Syndrome (AS), may be able to lead independent lives but still be awkward in their social interactions. Other, more rare autism spectrum disorders include Rett Syndrome (RS), which affects mostly girls, and Childhood Disintegrative Disorder (CDD), which affects mostly boys; in both cases, there is a period of normal development before the onset of autistic symptoms. Pervasive Developmental Disorder-Not Otherwise Specified may be diagnosed when a child has autistic symptoms but does not fit into another Autistic Spectrum Disorder diagnosis.