Genetic disorders may overlap in terms of their clinical expression and all mutations are not yet totally disclosed. We report herein two cases of RASA 1 syndrome mimicking hereditary hemorrhagic telangiectasia (HHT). The two cases illustrate a phenotype presentation of CM-AVM1 overlapping with the classical description of HHT. Indeed, visceral involvement and nasal telangiectasias in case 1 and pulmonary arteriovenous malformations, nasal telangiectasias and typical cutaneous telangiectasias in case 2 were suggestive of HHT. Genetic tests did not find any mutation in the three classical genes [Endogline, Activin receptor-like kinase 1 or Mothers against decapentaplegic homolog], but identified in both cases a RASA1 mutation. The physicians should consider this mutation when usual genes of HHT are not affected. (https://jmg.bmj.com/content/early/2020/09/07/jmedgenet-2019-106792 )