Erdheim-Chester disease (ECD) is a rare multisystemic disease characterized by the infiltration of multiple organs by foamy CD68 + CD1a- histiocytes (resident mononucleate macrophages) and “Touton” cells (multinucleated giant cells), which are typical of lesions with high lipid content, such as fat necrosis, xanthomas, and xanthelasmas. The gene discussed is CD1A; the disease is familial atrioventricular septal defect.