XBP-1 expression. Immunohistochemical localization of XBP-1 expression (brown color) in tumor cells surrounding the CNS tumor vasculature in a hematoxylin-stained tumor. (A) Transverse view; arrow points to a blood vessel. (B) Longitudinal view of vessel; XBP-1 immunoreactivity of tumor cells distinguishes them from endothelial cells. (C) Intense XBP-1 immunoreactivity exhibited by a dense population of tumor cells growing around a tumor vessel at the infiltrating edge of a different CNS lymphoma tumor specimen. (D) There was minimal XBP-1 immunoreactivity in a representative specimen of large B-cell lymphoma isolated from a lymph node. When assessed by immunohistochemistry, the pattern of BCL-6 expression was diffuse within CNS lymphomas, not differentially related to the tumor vasculature (not shown). Original magnification × 200 for all panels.