MMP13 knockout increases mammary tumor metastasis to lung. E0771 mouse mammary tumor cells were implanted into the mammary fat pad of congenic female C57BL/6 WT or MMP13 KO mice as described in Methods. At day 28 post-implantation, lungs were excised and processed for H&E staining for analysis of metastases. Representative images from the (A) WT and (B) MMP13 KO animals show increased metastastic burden (see arrows indicating metastases) in the MMP13 KO group. (C) Metastases/cm2 were counted and averaged from 10 equidistant sampling step sections through both lobes of the lung per animal from a cohort of WT (n=6) and MMP13 KO (n=4) mice, demonstrating that MMP13 KO mice had nearly double the number of lung metastases compared to their WT counterparts (*p < .006).