Can genome sequencing help to reveal the cause of an intellectual disability? how can an 'unknown cause' of it be eventually clarified?

In some cases: The advances in gene sequencing has provided some information on previously unexplained problems. Many autistic children,have evidence of deleted or added nonsense information on a variety of positions within the code. This represents a significant advance in the understanding of this form of impairment. Many with the problem will remain unexplained for a long time to come.

Yes: Causes of Intellectual Disability are: Chromosomal-30%, CNS malformations-15%, Congenital syndromes-5%, Metabolic-5%, Acquired-15%, Unknown-25-38%. Genetic testing revealed cause in 50% of these cases. Notably numerous other tests are important to diagnosis as well. Recent advances in cytogenetic & molecular genetic testing will lead to improved diagnosis & lower the % of unknowns in future.