Immunoreactivity for endoplasmic reticulum stress-markers in regenerating skeletal myofibers of myositis patients. Serial cryosections from biopsies of (a) to (c) a Group 0 subject (S6) and (d) to (i) a Group I myositis patient (P2), who displayed a high degree of muscle regeneration, were stained with indirect immunoperoxidase with antibodies for embryonic skeletal myosin heavy chain (εMy), Grp94, calreticulin (CRT), CHOP, complement 9 (C9) and ubiquitin (ubi). Thin arrows in (b) and (c) indicate interstitial cells reacting with anti-Grp94 and anti-caltreticulin antibodies. Thick arrows in (d) to (i) indicate a regenerating muscle fiber (εMy-positive), which stained positively for Grp94 (e), cytosolic CHOP (f), calreticulin (g) and ubiquitin (f). Thick arrowheads in (f) and (i) indicate a regenerating muscle fiber, which displayed nuclear CHOP (f) and ubiquitin (i) immunoreactivity. Arrowheads in (d) to (i) indicate a regenerating muscle fiber, which displayed positive immunoreactivity for Grp94 (e) and calreticulin (g), and both cytosolic and nuclear CHOP (f) and ubiquitin (i) staining. Double arrowheads in (d) to (h) indicate the presence of positive C9 immunoreactivity, a marker of necrosis (h), in a regenerating muscle fiber, which displayed positive immunoreactivity for Grp94 (e). Bar: 100 μm.