The 15q11.2 BP1-BP2 region is found duplicated or deleted in people with cognitive, language, and behavioral impairment. We report on a family (a father and 3 male twin siblings) that presents with a duplication of the 15q11.2 BP1-BP2 region and a variable phenotype: the father and the fraternal twin are normal carriers, whereas the monozygotic twins exhibit severe language and cognitive delay as well as behavioral disturbances. The genes located within the duplicated region are involved in brain development and function, and some of them are related to language processing. The probands' phenotype may result from changes in the expression level of some of these genes important for cognitive development.
Keywords: Cognitive delay; Copy number variations; Language impairment; Microduplication 15q11.2 BP1–BP2; Variable penetrance.