This unpredicted scenario was further confirmed by the identification of activating pathogenic variants of HRAS, a proto‐oncogene frequently mutated in cancer, as the molecular cause of Costello syndrome (CS; MIM 218040), a disorder related to NS characterized by an overall more severe clinical presentation and predisposition to certain malignancies (Aoki et al., 2005). Here, HRAS is linked to Cowden syndrome 1.