Comparison of thymic histology and RBTN-2 expression in MSH2 ^−/− mice before the development of lymphoma and MSH2 ^−/− mice with thymic lymphoblastic lymphoma (A-D). (A) Thymic histology from 8-week-old MSH2 ^−/− mouse before the development of lymphoma, original magnification (OM) × 500. (B) Thymic tumor histology from 4-month-old MSH2 ^−/−, OM × 500. Note the uniform morphologic appearance of the tumor cells with fine nuclear chromatin, inconspicuous nucleoli, scant cytoplasm, and numerous mitotic figures consistent with lymphoblastic lymphoma. (C) RBTN-2 protein expression by immunohistochemistry in thymus of MSH2 ^−/− mouse before the development of lymphoma, OM × 1,000. (D) MSH2 ^−/− thymic tumor, OM × 1,000. Immunohistochemistry showed strong RBTN-2 protein expression in thymic tumor only.