Although genome-wide linkage scans and association studies have provided some promising leads in identifying genes that contribute to autism, no clear links have been established. This study builds on genomic data gathered in cohorts of extended families in both Iceland and the U.S. in which one or more family members have an autism diagnosis. The genome-wide linkage scans in these cohorts have revealed a number of suggestive gene loci for autism or autism-related endophenotypes, including quantitative trait loci (QTLs). Although not necessarily genes themselves, QTLs are stretches of DNA that are closely linked to the genes that underlie a specific trait. This study will phenotype additional relatives in Iceland and the U.S. and analyze both cohorts in parallel, performing the same QTL analysis, followed by fine mapping and narrowing regions of shared linkage. The focus will be on the isolation of genes within loci that are supported in both populations.