Muscle pathology. The first biopsy was performed 3 months after ATO treatment withdrawal and demonstrated decreased COX levels (A) and numerous lipid droplets in muscle fibers (B-C). The second biopsy was performed 6 months after ATO treatment was interrupted and demonstrated a dramatic improvement in COX levels (D) and a marked decrease in size and number of lipid droplets in muscle fibers (E-F). Frozen sections stained with COX (panels A,D, original magnification ×40); oil red O (panels B,E, original magnification ×40); and semithin sections stained with toluidine blue (panels C,F, original magnification ×100). For image acquisition information please see supplemental Methods.