Finally it should be emphasized that, just as in the general population, there is variance in ability in Down syndrome resulting from genetic differences and differences in the environments through which genes act. Individuals with Down syndrome inherit a full set of chromosomes from their parents, as typically developing children do, along with the extra chromosome material. They also experience a wide range of environments at home and in school that will contribute to their literacy outcomes. There is an urgent need for longitudinal studies that follow the development of literacy in Down syndrome from before the start of formal schooling and that map relations between their general cognitive abilities, language and reading skills. Such studies should also investigate environmental influences on literacy. Findings from these studies may inform individual difference in responsiveness to intervention in Down syndrome.