Allelic loss (LOH) mapping and sequence analysis were conducted for gamma-ray induced mouse thymic lymphomas and a novel tumor suppressor gene, Rit1/Bcl11b, on chromosome 12 was isolated. Bi-allelic changes were found in 17 of the 66 p53-proficient lymphomas with Rit1 LOH but in only 2 of the 54 p53-deficient lymphomas. This suggests an association between the presence of functional p53 and inactivation of the Rit1 gene in the lymphoma development. Introduction of Rit1 into HeLa cells lacking Rit1 expression suppressed cell growth. These results indicate that loss-of-function mutations of Rit1 contribute to mouse lymphomagenesis and possibly to human cancer development.