Morphological, immunophenotypic, and genetic features of a LBCL with IRF4 rearrangement (case D62). Architecture effacement by an atypical lymphoid proliferation with nodular growth pattern (A; hematoxylin and eosin) that corresponds to expanded follicles with highly disrupted follicular dendritic cell meshwork (B; CD21). The atypical cells were negative for CD10 (C) and positive for BCL6 (D). BCL2 was positive in the accompanying reactive T cells but negative in the tumor (E), which exhibits a high proliferation rate (F; Ki67). The immunohistochemical study for IRF4/MUM1 (G; MUM1) shows strong and diffuse positivity in the neoplastic proliferation, and FISH with IRF4 break-apart probe shows a signal constellation of 1 colocalization (yellow arrow) and 1 split signal (red and green arrows) consistent with the gene rearrangement (H). Original magnification ×100 (A), ×40 (B-G).