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Specific reading disability is a complex phenotype that is under both genetic and environmental influences. There is evidence for at least one major gene, which may be detectable by parametric linkage analysis, but detection of other quantitative trait loci may require nonparametric methods. Phenotype definition may also be critical in identifying genes that affect different components of the reading process. Current research from two separate laboratories supports the localization of one gene influencing reading disability to the histocompatibility region of chromosome 6p and suggests that another gene may be located on chromosome 15. Actual identification of these and similar genes is complicated by reduced penetrance and phenocopies in families, so that delineation of the critical region using crossover analysis is problematic. This difficulty can result in the identification of many candidate genes that need to be evaluated through combinations of association analysis, functional analysis, and mutation assays.