Development of lymphoma in transgenic mice. (A) and (B) depict the gross appearance of malignant lymphomas developed in transgenic mice. In (A), the tumor (T) was abdominal and compressed, but did not invade the intestines (I). In (B), the tumor (T) involved the thymus and invaded lungs (L) and heart (H). (C) and (D) show representative histologic sections of the tumors shown in (A) and (B), respectively. Diffuse small lymphocytic (low-grade) lymphoma is seen in the abdominal tumor, and a more aggressive lymphoma with open chromatin (lymphoblastic-like) is seen in the thymus tumor. (E) Shows lymphoma invading skeletal muscles. (F ) Shows another pattern of lymphoma observed in abdominal tumors with small cells and condensed chromatin.