There are 28 distinct types of MMPs in vertebrates, in which 23 are expressed in human tissue. The types of MMPs are organized into 5 categories. One category is non-furin regulated MMPs (MMP-1, -3, -7, -8, -10, -12, -13, -20, and -27). The second category is MMPs consisting of three fibronectin-like inserts in the catalytic domain (MMP-2 and -9). The third category is MMPs anchored to the cellular membrane by a C-terminal glycophosphatidylinositol (MMP-11, -17, and -25). The fourth category is MMPs with a transmembrane domain (MMP -14, -15, -16, -24). The last category are all non classified MMPs (MMP -19, -21, -23, -26, and -28).