MMP13 knockout increases Collagen I levels in E0771 mammary tumors. WT and MMP13 KO mice were implanted with E0771 mammary tumors as described. Following excision of the primary tumor, tumor boundary regions were assessed for Collagen I levels by immunochemistry. Immunofluorescence signal was captured by two-photon excitation microscropy of fields of view (FOV) from random tumor boundary regions. Z-stacks from each FOV were maximum intensity projected and background subtracted, and fluorescent intensities from the resultant images were quantified with ImageJ and then expressed as mean anti-Collagen I immunofluorescence ± SEM from n=16 (WT) and n=14 (MMP13 KO) tumor FOVs from the same cohort of animals. Collagen I levels in tumor peripheries were significantly increased in MMP13 KO versus WT mice (*p < .0004).