Analysis of RTK activation with p-Y–specific antibodies. Immunohistochemical analysis with antibodies specific for p-Y-720-PDGFRA (A) and p-Y-680/681-TRKA and -TRKB (B), using alkaline phosphatase and Fast Red as substrate, showed considerable HRS cell–specific staining in patients expressing the respective RTKs. In positive patients, only a fraction of HRS cells (approximately 30%-70%) was stained. Positive and negative cells were intermingled. Immunostainings with a pan-p-Y antibody using horseradish peroxidase and diaminobenzidine (DAB) as substrate revealed elevated levels of p-Y contents, comparable to tumors with activated RTKs, in the HRS cells of several patients (C-E). The same patient with HL is shown in panels E and F. (E) HRS cells are positive, (F) and pretreatment of the section with T cell phosphatase abolished staining completely. All magnifications except panel D are 200×, with magnifications in inserts at 400×. Magnification in panel D is 100×.