Extracranial arteriovenous malformations (eAVMs) contain a heterogeneous population of both CD31+ and CD31- vascular endothelial cells (eAVM-ECs), the latter of which line grossly dilated vessels and are surrounded by an expanded perivasculature of PDGFRβ+ mural cells (eAVM-MCs). The gene discussed is PECAM1; the disease is arteriovenous hemangioma/malformation.