hMSH2 expression in reactive secondary B-cell follicle in lymphnode (A), spleen (B), appendix (C), and palatine tonsil (D). hMSH2 is a nuclear located protein selectively expressed by follicular germinal center B cell; the mantle, paracortical, and marginal zones appear hMSH2 negative with the exception of large secondary blasts that are crossing the mantle. (E) AIDS-related PCNSL; (F) AIDS-unrelated PCNSL. Most neoplastic cells show a nuclear staining pattern with anti-hMSH2 MoAb. Paraffin-embedded tissue section, ABC-px immunostaining, hematoxylin counterstain. Original magnification × 100 (A), (B), × 250 (C through F).