MMP9 and neoplasm: Low pH activates and triggers the secretion of proteolytic enzymes including matrix metalloproteinase‐2 (MMP‐2), MMP‐9, a tissue serine protease, adamalysin‐related membrane protease, cysteine protease, cathepsin and gelatinase leading to the degradation and remodelling of ECM, so as to promote tumour invasion and progression.72