Genetic alterations that promote chromosome missegregation have been proposed to drive tumorigenesis through loss of whole chromosomes containing key tumor suppressor genes. To test this unproven idea, we bred Bub1 mutant mice that inaccurately segregate their chromosomes onto p53(+/-), Apc(Min/+), Rb(+/-), or Pten(+/-) backgrounds. Bub1 insufficiency predisposed p53(+/-) mice to thymic lymphomas and Apc(Min/+) mice to colonic tumors. These tumors consistently lacked the nonmutated tumor suppressor allele but had gained a copy of the mutant allele. In contrast, Bub1 insufficiency had no impact on tumorigenesis in Rb(+/-) mice and inhibited prostatic intraepithelial neoplasia formation in Pten(+/-) mice. Thus, Bub1 insufficiency can drive tumor formation through tumor suppressor gene loss of heterozygosity, but only in restricted genetic and cellular contexts.