Aims The causes of intellectual disability, which affects 1%–3% of the general population, are highly heterogeneous and the genetic defect remains unknown in around 40% of patients. The application of next-generation sequencing is changing the nature of biomedical diagnosis. This technology has quickly become the method of choice for searching for pathogenic mutations in rare uncharacterised genetic diseases.

Methods Whole-exome sequencing was applied to a series of families affected with intellectual disability in order to identify variants underlying disease phenotypes.

Results We present data of three families in which we identified the disease-causing mutations and which benefited from receiving a clinical diagnosis: Cornelia de Lange, Cohen syndrome and Dent-2 disease. The genetic heterogeneity and the variability in clinical presentation of these disorders could explain why these patients are difficult to diagnose.

Conclusions The accessibility to next-generation sequencing allows clinicians to save much time and cost in identifying the aetiology of rare diseases. The presented cases are excellent examples that demonstrate the efficacy of next-generation sequencing in rare disease diagnosis.