Histopathologic analysis of organ localizations. Histopathologic analysis of the kidney from Rag2^−/−γ_c^−/− male mice injected subcutaneously with MEC1 cells showed discrete collections of tumoral lymphocytes (A and B: H&E staining, magnification ×200 and ×400, respectively). These neoplastic nodules displayed homogeneous distinctive immunoreactivity for human CD19 (C), CD20 (D), and CD23 (E), with a substantial proliferation index (F: Ki67).