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The prevalence of autism spectrum disorders (ASD) has been high worldwide,reaching 1/59 children in the United States as reported by the Centers of Disease Control and Prevention.Since genetic components play a major role in ASD [1],it is astonishing that the occurrence of ASD would be this high probabily due to genetic causes.It is worthy to note that autistic phenotypes of ASD patients show great diversity.The severity of autistic symptoms may be correlated with whether genetic mutations affect neural development.Thus,we argue that the prevalence of severe ASD may be much lower than the common ASD usually reported.In clinic and neurobiological fields,the ASD candidate genes are usually critical genes whose loss-of-function will affect neural development dramatically.In the following sections,I will focus on the recent progress on clinical diagnosis of severe ASD,as well as genetic and neurobiological studies.