Immunohistochemical detection of HLA-DR and relationship to gene expression. (A) Photomicrograph demonstrating a low HLA-DR-expressing PMBCL case, case 691. Staining of endothelial cells and some histiocytes serves as an internal positive control. (B) Photomicrograph demonstrating a high HLA-DR-expressing PMBCL case, case 824. Original magnification for both × 100. (C) Graph of the relationship between results of immunohistochemistry (IHC) of HLA-DR in percentage of positive tumor cells (y-axis) and average MHC class II gene expression from microarray results (x-axis). Pearson correlation R = 0.68. A trend line is shown to help judge the extent of correlation. Arrows show where the cases pictured above are located in the graph.