MMP9 and breast cancer: Another important class of pubertal phenotypes is caused by loss- or gain-of-function of matrix remodeling enzymes such as matrix metalloproteinases (MMPs), which regulate ductal invasion and branching via their ability to sculpt the ECM45, 46, and Fgfr1/2 stimulation has been shown to induce the expression of Mmp3 and Mmp9 in several breast cancer and immortalized mammary epithelial cell lines47–51.