Uniform criteria for evaluation of RS COX-2 staining. Representative immunohistochemistry cases from the tissue microarrays of negative RS COX-2 staining (A), low positive RS COX-2 staining (B), or high positive RS COX-2 staining (C). All cases are presented at a 40× magnification. Thin arrows indicate RS cells; arrowheads, histiocytes; thick arrows, plasma cells. Colon mucosa was used as a positive control for COX-2 expression (D).