Mutation of Id3 results in lymphoma. (A) Splenomegaly (indicated by yellow arrow) and hepatomegaly (indicated by green arrow) in 1 representative Id3^−/− mouse. (B) Massive lymphadenopathy in a representative Id3^−/− mouse (highlighted with dotted lines). (C) Comparison of spleen and lymph nodes between Id3^−/− (> 12 months) and age-matched WT control mice. The numbers indicate the total number of spleen cells. (D-G) H&E staining of the spleen (D), liver (E), or lungs (F) in a representative Id3^−/− mouse with lymphoma. Magnification ×10. Arrows indicate neoplastic cell infiltration. (G) High magnification of tumor cells in liver (×10). (H) Immunohistochemical staining of tumor cells with CD3-specific antibody (T cell, top right) and B–cell–specific antibody (B220, B cell, bottom right). Arrows indicate positive cells. Data shown in left column are same tissues stained with isotype control antibodies (rat immunoglobulin G2a for T cell; rabbit immunoglobulin G for B cell) with no positive staining.