Multifocal capillary malformation (CM) is the cardinal feature of patients with autosomal dominant inherited RASA1 mutations, and represents a strong phenotypic marker, present in 97% of mutation carriers.1 These CMs are ‘red flags’, signalling the possible association with an arteriovenous malformation (AVM) or arteriovenous fistula (AVF). The gene discussed is RASA1; the disease is cutaneous mastocytosis.