Recurrent chromosomal rearrangements at BCL11B are found in human hematopoietic malignancies mostly of T-cell origin. However, it is unclear how this disruption contributes to oncogenesis, because the majority of leukemias express BCL11B from an undisrupted allele. Here, we show that Bcl11b(+/-)p53(+/-) mice exhibited greater susceptibility to lymphomas than Bcl11b(+/+)p53(+/-) mice but most lymphomas retained and expressed the wild-type Bcl11b allele. This strongly suggests that Bcl11b is haplo insufficient for suppression of thymic lymphoma development in mice of the p53(+/-) background, a situation in which functional loss of only one allele confers a selective advantage for tumor growth. The haploinsufficiency is further supported by that Bcl11b(+/-) mouse embryos were impaired in thymocyte development and survival. These results indicate relevance of BCL11B aberration to human leukemogenesis. (c) 2007 Elsevier Inc. All rights reserved.