Dr. Nicole de Leeuw, a clinical laboratory geneticist in the Department of Human Genetics of the Radboud University Nijmegen Medical Centre in Nijmegen, and colleagues performed genome-wide SNP array analysis in 6,500 patients and 1,874 parents. The patients had intellectual disability and/or congenital abnormalities, and the parents of those in whom an aberration was detected were tested in a similar way to determine whether they had the same aberration as their child. Mosaic aberrations, where both genetically normal and abnormal cells are present in an individual, were not only found in one in every 300 patients, but in one in every 270 parents as well. "These abnormalities occurred more frequently than we had expected", said Dr. de Leeuw. "Armed with this knowledge, we can try to understand not only why, but also how genetic disease arises in individuals, and this can help us to provide better genetic counselling."