HLA-DR is not expressed in PTLD biopsies compared with tonsil biopsies. PTLD, tonsils, and HD sections were used for IHC assays and the nuclei counterstained with hematoxylin. (A) Positive signals of HLA-DR were observed as a brown color in tonsil biopsies but not in PTLD biopsies. (B) Positive signals of HLA-DR were observed as a brown color in EBER− biopsy (case 11) but not in EBER+ biopsy (case 1). The nuclei are stained blue. Original magnification, ×200. Scale bar indicates 50 μm.