Tracing Autism Traits in Large Multiplex Families to Identify Endophenotypes of the Broader Autism Phenotype.Families comprising many individuals with Autism Spectrum Disorders (ASD) may carry a dominant predisposing mutation.  We implemented rigorous phenotyping of the "Broader Autism Phenotype" (BAP) in large multiplex ASD families using a novel endophenotype approach for the identification and characterisation of distinct BAP endophenotypes.  We evaluated ASD/BAP features using standardised tests and a semi-structured interview to assess social, intellectual, executive and adaptive functioning in 110 individuals, including two large multiplex families (Family A: 30; Family B: 35) and an independent sample of small families (n = 45).  Our protocol identified four distinct psychological endophenotypes of the BAP that were evident across these independent samples, and showed high sensitivity (97%) and specificity (82%) for individuals classified with the BAP.  Patterns of inheritance of identified endophenotypes varied between the two large multiplex families, supporting their utility for identifying genes in ASD.
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