MMP9 and triple-A syndrome: One of the causes of AAA formation may be the degradation of elastin fibers by increasing the amount and activity of enzymes belonging to different classes, including cysteine proteases (cathepsin), serine proteases (e.g., neutrophil and pancreatic elastase) and proteins from the family of metalloproteinases of the extracellular matrix, in particular MMP-1, MMP-2 and MMP-9.