"Mental retardation, like most common neurodevelopmental and psychiatric diseases, shows a strong genetic component, but these underlying genetic causes remain largely unknown. For a long time it was hypothesized that these kind of common diseases are mainly caused by common inherited genetic variants with reduced penetrance. In contrast to this common variant-common disease hypothesis, I here hypothesize that a large proportion of this so-called “missing heritability” for conditions such as mental retardation, schizophrenia, and autism lies in de novo genetic variation that is rapidly eliminated from the population because individuals with such diseases have severely compromised fecundity.My previous work using microarrays has already demonstrated de novo genomic copy number variations in mental retardation and in schizophrenia. However, microarrays do not allow us to capture the most common form of de novo mutations, those occurring at the nucleotide level. Technological innovations now for the first time allow us to comprehensively study the entire genome of an individual for genomic variations at all levels. In this project I will explore the de novo mutation hypothesis in whole exome and whole genome sequence data from patients with mental retardation. I will optimize and apply whole genome sequencing strategies using patient-parent trios, both in rare mental retardation syndromes as well as common forms of mental retardation. Guidelines for pathogenicity will be established by computational studies aimed at unraveling genotype-phenotype correlations in these family-based genome sequence type datasets.This project will contribute significantly to resolving the genetic causes of reproductively lethal disorders such as mental retardation, provide critical knowledge on the frequency and consequences of de novo mutations in our genome and help to establish medical genome sequencing as a routine diagnostic approach."