MME and neoplasm: A biopsy of the retroperitoneal tumor showed proliferation of small, atypical lymphoid cells with high nuclear-to-cytoplasm ratio (Figure 2A; Supplementary Figure S2A), positive for CD10, CD19, CD79a, BCL-2 (Supplementary Figure S2B), BCL-6 (Supplementary Figure S2C), and c-MYC (Supplementary Figure S2D) by immunohistochemistry.