MMP9 and neoplasm: The integrin β4/FAK/SOX2/HIF-1α signaling pathway also regulates the metastasis of tumor cells [47]; integrin β4 overexpression triggers increases in the expression of matrix metalloproteinase-2 (MMP-2) and MMP-9 through the ERK1/2 pathway, the degradation of the extracellular matrix, and the invasion ability of tumor cells [50,51] (Figure 1).