Phenotype of transgenic mouse tumors. (A) Cervical lymph node tumor cells were harvested from tumor-burdened mice and stained with B220-APC (y-axis) and IgM-PE or CD43-PE (x-axis) and analyzed by flow cytometry. The percentage of cells that fall within each quadrant is indicated. Both IgM⁺ (n = 7) and IgM⁻ (n = 5) λ-MYC tumors are observed, and an example of each is shown. All LMP2A/λ-MYC tumors are IgM⁻ (n = 18; see also summarized quadrant data in Tables 3 and 4). (B) Cervical lymph node tumors were isolated from tumor-burdened mice and fixed in 10% buffered formalin phosphate followed by paraffin embedding. Tumors were then sectioned and stained with hematoxylin and eosin. Note the “starry sky” pattern that is also characteristic of human Burkitt lymphoma tumors.