Immunohistochemical validation of some overexpressed genes using tissue microarrays. Immunostaining of 2 representative cases of ALK⁺ (A-B) and ALK⁻ (C-D) ALCL. Malignant cells in the ALK⁺ patient strongly express C/EBPβ (A-B). Immunoperoxidase staining, magnified 50 × and 1000 ×. In the patient negative for ALK protein, only reactive histiocytes show strong nuclear staining (C-D). Immunoperoxidase staining, magnified 50 × and 400 ×. Images were captured and processed as described in Figure 2, except that an HC PL Fluotar 5×/0.15 NA objective was used for panels A and C.