E2F4 and neoplasm: Three of these mice exhibited lymphoma with modest spleen enlargement and isolated lymph node enlargement, similar the uncommon late onset lymphomas that occasionally develop in Eμ-myc mice wild type for E2F. The ten remaining E2f4−/− Eμ-myc mice displayed an atypical tumor phenotype that was only rarely noted in E2F wild type Eμ-myc mice (10 of 17 E2f4−/− mice compared to only 3 of 79 E2f4+/+ mice).