CD68 and familial atrioventricular septal defect: 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.