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The Twin Research Registry (TRR) at SRI International is a community-based registry of twins established in 1995 by advertising in local media, mainly on radio stations and in newspapers. As of August 2012, there are 3,120 same- and opposite-sex twins enrolled; 86% are 18 years of age or older (mean age 44.9 years, SD 16.9 years) and 14% less than 18 years of age (mean age 8.9 years, SD 4.5); 67% are female, and 62% are self-reported monozygotic (MZ). More than 1,375 twins have participated in studies over the last 15 years in collaboration with the University of California Medical Center in San Francisco, the University of Texas MD Anderson Cancer Center, and the Stanford University School of Medicine. Each twin completes a registration form with basic demographic information either online at the TRR Web site or during a telephone interview. Contact is maintained with members by means of annual newsletters and birthday cards. The managers of the TRR protect the confidentiality of twin data with established policies; no information is given to other researchers without prior permission from the twins; and all methods and procedures are reviewed by an Institutional Review Board. Phenotypes studied thus far include those related to nicotine metabolism, mutagen sensitivity, pain response before and after administration of an opioid, and a variety of immunological responses to environmental exposures, including second-hand smoke and vaccination for seasonal influenza virus and Varicella zoster virus. Twins in the TRR have participated in studies of complex, clinically relevant phenotypes that would not be feasible to measure in larger samples.
Rather than being separate and distinct, learning disabilities are often comorbid in children, suggesting that there may be overlaps in deficits and etiologies. Through the identification of genes influencing processes that are important to learning, new developments in genetic analysis may help define alternate ways of conceptualizing different types of learning disabilities based on the genes and endophenotypes that are involved in each one. There has been evidence since the turn of the last century that reading disability (RD) occurs in families, and twin studies have shown heritabilities around 0.56. Although the definitions of RD and language impairment (LI) are based on measures that are very different, the disorders share some similarities; the heritabilities for deficits are similar, males are affected more often than females, and young children with LI are at greater risk for RD. Molecular genetic studies support common genetic influences on RD and speech sound disorder (SSD).