While Langerhans' cell histiocytosis exhibits the proliferation of CD1a(+), Langerin(+), S100(+) Langerhans' dendritic cells, ECD is hypothesized to originate from the proliferation and migration of CD68(+), CD1a(−) non Langerhans' histiocytes of monocyte-macrophage descent. The gene discussed is CD1A; the disease is familial atrioventricular septal defect.