Among rare diseases, the KANSL1 haploinsufficiency syndrome is one of the most common conditions. It can be caused by a small deletion on chromosome 17, removing the KANSL1 gene and other genes, or by a mutation in KANSL1. By analysing a large cohort of patients, we found that the degree of cognitive impairment is very mild in most. More importantly, although chromosome deletions are more frequent than KANSL1 mutations, we observed that KANSL1 is the major gene for this condition. It palys a role in regulating the activity of many genes, by a DNA process defined as “histone acetylation”. Drugs with acetylating activity might eventually be used in targeted therapy of this condition. (By Prof. Marcella Zollino, http://jmg.bmj.com/content/early/2015/09/30/jmedgenet-2015-103184 )