The number of cells expressing extranuclear high-mobility group chromosomal box protein 1 (HMGB1) is quantitatively comparable to the number of cells expressing tumor necrosis factor (TNF) and interleukin-1-beta (IL-1β) in arthritic joints. Representative micrographs illustrating immunohistochemical staining of synovial tissue from an arthritic animal at day 21 after immunization. Sequential cryocut sections were analyzed for expressions of HMGB1 (a), IL-1β (b), and TNF (c). Abundant expressions were demonstrated for all three cytokines. (d) A section stained with an irrelevant isotype-matched control. Original magnification ×125.