(Visit: www.uctv.tv) The transition of autism from a diagnostically inclusive, behaviorally-defined disorder to an etiologybased classification system mirrors the history of medical genetics and mental retardation. Beginning in the 1960s, geneticists used physical dysmorphology, biochemical and other laboratory data to delineate the heterogeneity within multiple behaviorally-based phenotypes. To accomplish a similar shift for autism, we identified "phenotypic features" that are present in some individual and relatively discrete, quantifiable and pathophysiologically relevant. Morphologic, physiologic, medical and family history all provide helpful biomarkers that distinguish subsets of autism. Proof that a biomarker splits out an etiologically distinct subset depends on showing differences in outcome measures, clinical course, response to therapy, genetic indicators (sex ratio, recurrence risk) and ultimately identification of separate genes or genetic pathways. Series: "MIND Institute Lecture Series on Neurodevelopmental Disorders" [10/2013] [Health and Medicine] [Show ID: 24421]