Approximately 50% of CM-AVM cases are caused by pathogenic germline variants in RASA1, which encodes the p120-RAS GAP (Eerola et al., 2003; Revencu et al., 2008), with the remaining caused by other RAS/MAPK pathway genes such as EPHB4 (Amyere et al., 2017). The gene discussed is RASA1; the disease is cutaneous mastocytosis.