MMP9 and head and neck squamous cell carcinoma: The expression of a group of metalloproteinases, including MMP-1, MMP-2, MMP-3, MMP-7, MMP-8, MMP-9, MMP-10, MMP-11, MMP-13, MMP-14, and MT-1 MMP [9], was shown in the head and neck squamous cell carcinomas.