In addition, histological examination and nuclear grading of the tumors revealed no significant differences in the percentages of tumors that had progressed to grade III, late-stage carcinoma (Fig. 4C, 8% of the MMTV-PyMT;Mmp13−/− vs. 18% of MMTV-PyMT;Mmp13+/+ tumors). This evidence concerns the gene MMP13 and carcinoma.