MMP9 and head and neck squamous cell carcinoma: A number of studies aimed to explore the association of protein expression and single nucleotide polymorphisms (SNP) of MMP-1, MMP-2 (−1306 C/T), MMP-3 (−1171 5A > 6A), and MMP-9 (−1562 C/T) promoter in OSMF and HNSCC.