A substantial portion of cells expressing cytoplasmic high-mobility group chromosomal box protein 1 (HMGB1) are also ED1-positive. (a) Micrograph illustrating a high magnification of inflamed synovial tissue stained with hematoxylin (arrows). (b) The intranuclear HMGB1 staining (Oregon green) of resident cells is evident. A substantial portion of cells express extranuclear HMGB1 (arrows). (c) A substantial portion of cells expressing extranuclear HMGB1 were also ED1-positive (Red Alexa 546) (arrows). ED1 is a surface membrane antigen expressed on rat macrophages, monocytes, and dendritic cells. Original magnification ×800.