We report a novel genetic mechanism underlying Auriculocondylar syndrome (ARCND), a rare disease that mainly affects jaw and outer ear development. In a Brazilian family we identified a large duplication of DNA within the HDAC9 gene that contains elements involved in regulation of the neighbouring gene, TWIST1. We found higher levels of HDAC9 and TWIST1 in ARCND patient neural crest cells, which are the cells that give rise to the craniofacial structures compromised in this disorder. In addition, altered cell migration and ossification were also observed. Our findings show that this duplication is pathogenic and that deregulation of TWIST1 gene expression contributes to the disease. (By Dr. Maria Rita Passos Bueno, https://jmg.bmj.com/content/early/2021/11/07/jmedgenet-2021-107825 )