MMP9 and Duchenne muscular dystrophy: MMP-9 is secreted from both skeletal muscle and cardiac muscle into the extracellular matrix, where it exacerbates skeletal muscle and cardiac muscle phenotypes in DMD by contributing to increased inflammation and muscle damage and reduced skeletal and cardiac muscle function (Kherif et al., 1999; Li et al., 2009; Ogura et al., 2014).