Expression of PTPRK in primary HL biopsies. Immunohistochemistry significantly demonstrates the frequent down-regulation of PTPRK expression in HRS cells (→) of EBV-positive cases of HL. Shown is representative staining for 2 EBV-negative cases (left panel) and 2 EBV-positive cases (right panel). Subsets of surrounding lymphoid cells were PTPRK positive and provided an internal positive control. Images were acquired using Nikon TE2000 with 60×/1.4 NA oil-immersion lens (Nikon, Kingston-upon-Thames, United Kingdom). Cells were stained with DAB immunoperoxidase and counterstained with hematoxylin. Images were captured with a Nikon Coolpix 2100, and Paint Shop Pro 8.0 (Jase Software, Maidenhead, United Kingdom).