Capillary malformation – arteriovenous malformation is an autosomal dominant disorder, characterised by capillary malformations and an increased risk of fast-flow lesions. It is caused by mutations in two genes: RASA1 or EPHB4. Around 25% of the patients remain unexplained. In this study, we show that some of the so called “negative cases” are caused by mosaic RASA1 mutations. Those patients present classical phenotype, even in case of low-level mosaicism, with an increased risk to pass the mutation on to offspring. Finally, our results underscore the pathophysiological mechanism necessitating a second-hit to explain the multifocality of vascular lesions in this disorder. (By Dr. Nicole Revencu, https://jmg.bmj.com/content/early/2019/07/12/jmedgenet-2019-106024 )