Mental retardation, which is usually defined as an IQ score below 70, occurs in approximately 1% of the population. It is the most frequent cause of severe life-long disability in Canada and its burden on affected families and society is considerable. This project evaluated much more sensitive new technology (high-resolution array genomic hybridization) to detect chromosome abnormalities in people with mental retardation as an alternative to current practices. Their results showed that this new technology identified the cause of mental retardation in at least twice as many cases as conventional cytogenetic analysis, which is the current clinical standard. This was a cross-Canada study, including more than 50 clinical geneticists and cytogeneticists from 18 genetic centers in 7 provinces. Samples from more than 200 families had been collected and genetic counselling had been provided to more than a dozen families by the end of the project.