This paper reports data from large North American and European cohorts that were ascertained for the 16p11.2 deletion (break points 4-5) without regard to age or diagnosis. The deletion was previously reported to be among the most frequent known genetic etiologies of autism spectrum disorder and related neurodevelopmental disorders. Here the authors describe the medical, neuropsychological and behavioral phenotypes in carriers of this deletion compared to intrafamilial non-carrier controls. They report general clinical findings such as head circumference and rates of epilepsy as well as how the deletion impacts IQ, behavior and BMI in a consistent quantitative manner. (By Sébastien Jacquemont, http://jmg.bmj.com/content/49/10/660 )