Developmental delay, learning difficulties, behavioural abnormalities and birth defects are often caused by gains or losses of genetic material.  Array comparative genomic hybridisation (aCGH) is a new technology which is more sensitive than previous technologies, and therefore detects these causative changes and provides a diagnosis in more patients than was previously possible.  However, a(CGH) may also identify unexpected changes in genes associated with cancer predisposition. We describe our experience of these unexpected findings following testing of 4805 non-cancer referrals to our Genetics Centre and discuss the need for pre- and post-test genetic counselling and for long term follow-up to establish the cancer-causing relevance of these changes and  to help shape future policy. (By Dr Gabriella Pichert, http://jmg.bmj.com/content/early/2011/03/22/jmg.2010.087593.abstract?papetoc)