A detailed histological analysis revealed that tumor spectrum associated with the Mlh1 defect was modified in double-mutant mice, which developed only high-grade lymphomas compared to Mlh1 single knockout mice, which most frequently manifested high grade lymphoma (52%), but also lymphoid hyperplasia (19%) and other tumors (5%) (Fig. 5a–5d). This evidence concerns the gene MLH1 and lymphoma.