The propeptide (LOX-PP) domain of the lysyl oxidase proenzyme was shown to inhibit the transformed phenotype of breast, lung and pancreatic cells in culture and the formation of Her2/neu-driven breast cancer in a xenograft model. A single nucleotide polymorphism (SNP, rs1800449) positioned in a highly conserved region of LOX-PP results in an Arg158Gln substitution (humans). This arginine (Arg)→glutamine (Gln) substitution profoundly impaired the ability of LOX-PP to inhibit the invasive phenotype and xenograft tumor formation. To study the effect of the SNP in vivo, here we established a knock in (KI) mouse line (LOX-PPGln mice) expressing an Arg152Gln substitution corresponding to the human Arg158Gln polymorphism. Breast cancer was induced in wild-type (WT) and LOX-PPGln female mice beginning at 6 weeks of age by treatment with 7,12-dimethylbenz(a)anthracene (DMBA) in combination with progesterone. Time course analysis of tumor development demonstrated earlier tumor onset and shorter overall survival in LOX-PPGln versus WT mice. To further compare the tumor burden in WT and LOX-PPGln mice, inguinal mammary glands from both groups of mice were examined for microscopic lesion formation. LOX-PPGln glands contained more lesions (9.6 versus 6.9 lesions/#4 bilateral). In addition, more DMBA-treated LOX-PPGln mice had increased leukocyte infiltrations in their livers and were moribund compared with DMBA-treated WT mice. Thus, these data indicate that the Arg→Gln substitution in LOX-PP could be an important marker associated with a more aggressive cancer phenotype and that this KI model is ideal for further mechanistic studies regarding the tumor suppressor function of LOX-PP.