We engineered a mammary-specific knockout model for Brca1 deficiency that also lacks the majority of one chromosome 11 to determine whether tumor susceptibility loci reside on this chromosome that cooperate with the loss of Brca1 during mammary cancer formation. Brca1-deficient females that are haploinsufficient in 60 cM of chromosome 11 exhibited accelerated mammary tumorigenesis in comparison to Brca1 conditional knockout mice. On the histopathologic level, these tumors were either adenocarcinomas or benign, inflammatory lesions. Like human BRCA1-associated breast cancers, mammary carcinomas in this new mouse model were ERα-negative and of basal epithelial origin. Brca1 deficiency and haploinsufficiency in 60 cM of chromosome 11 caused widespread genome instability as determined by spectral karyotyping analysis. In addition to the verification of the long-range deletion event, the spectral karyotyping analysis revealed that the duplication of the genome and higher degree of aneuploidy occur rather late in tumor progression. Despite chromosomal rearrangements near the Trp53 locus as determined by fluorescence in situ hybridization, the Trp53 gene was transcriptionally active. The analysis of the coding sequence and expression pattern of p53 and p21 suggests that loss-of-heterozygosity of Trp53 caused by somatic mutations contributes to accelerated mammary tumorigenesis in this model.