Figure 2 depicts the conserved domains across all MMPs in their pro-enzyme form (Figure 2). Some secreted MMPs, such as MMP-9 and MMP-2, are highly expressed in MM and contain three fibronectin-like repeats in the catalytic domain [2,30]. In MMP-2 and MMP-9, the catalytic zinc is coordinated by three histidines and glutamic acid. The most recently discovered MMP-23 has a less typical structure, with immunoglobulin-like and cysteine-rich domains and a transmembrane domain linked to the N-terminus [30,35]. This evidence concerns the gene MMP9 and Miyoshi myopathy.