The TLX1/HOX11 homeobox gene was originally identified at the recurrent t(10;14)(q24;q11) translocation breakpoint, a chromosomal abnormality observed in 5–7% of T-cell acute lymphoblastic leukemias (T-ALLs). Proviral insertional mutagenesis studies performed on transgenic mice ectopically expressing TLX1/HOX11 in B lymphocytes (IgHμ-HOX11Tg mice) revealed the Ubr1 gene locus as a frequent site of proviral insertion, concomitant with accelerated development of diffuse large B-cell lymphoma. Insertion into this genomic region was confirmed by Southern blotting and by the ability to generate a polymerase chain reaction (PCR) amplicon across the viral–genome junction. Western immunoblot and semiquantitative reverse transcriptase–PCR analysis revealed downregulated expression of the Ubr1 gene product subsequent to viral integration. Loss or reduced levels of Ubr1 expression was associated with 5/14 spontaneous B-cell lymphomas in IgHμ-HOX11Tg mice and one of nine primary human T-ALLs. To gain mechanistic insight into the cooperativity between TLX1/HOX11 and Ubr1, IgHμ-HOX11Tg/Ubr1−/− mice were generated. IgHμ-HOX11Tg/Ubr1−/− mice exhibited a modest but statistically significant acceleration of disease onset relative to IgHμ-HOX11Tg/Ubr1+/− mice. Moreover, micronucleus assays to detect for chromosome missegregation were conducted and revealed increased presence of micronuclei in IgHμ-HOX11Tg/Ubr1−/− primary B lymphocyte cultures, and in both TLX1/HOX11-overexpressing T cell lines and fibroblast cultures following transfection with short interfering RNAs (siRNAs) targeting Ubr1. Karyotyping of primary B lymphocyte cultures revealed increased incidences of hypodiploid karyotypes. Finally, mitotic figures analysed from Ubr1 siRNA-transfected fibroblast cultures revealed no defects in chromosome congression to the metaphase plate, but increased incidences of atypical anaphase figures, including the development of anaphase bridges and lagging chromosomes. Based on these findings, we identify a synergistic role between TLX1/HOX11 overexpression and Ubr1 inactivation in promoting chromosome missegregation, permitting the accrual of additional chromosome losses and cytogenic abnormalities en route to malignancy.