NPM-ALK–induced lymphoid tumors. (A) Macroscopic appearance of subcapsular lymphomas metastatic to the kidney in animal GT4 showing bilateral multifocal involvement (original magnification × 1.5). (B) Hematoxylin and eosin-stained section of mesenteric lymphoma from mouse GT4 showing a uniform population of large immunoblastic cells with basophilic cytoplasm, centrally located nuclei, and distinct nucleoli (original magnification × 275). (C) Immunostaining with an anti-ALK antibody shows staining of the nucleoli as well as the cytoplasm of lymphoma cells, with heterogeneity in level of NPM-ALK expression evident among the tumor cells (original magnification × 275). (D) Immunostaining with an anti-CD3 antibody shows CD3⁻ tumor cells and some admixed CD3⁺ normal mouse lymphocytes (original magnification × 275). (E) Immunostaining of tumor cells is positive with an anti-IgM antibody, with infiltrating normal T lymphocytes serving as a negative internal control (original magnification × 275).