MMP9 and head and neck squamous cell carcinoma: Other genetic factors that are classified into similar polymorphism as MMP-1 have also been reported, such as Cyclin D1 (CCND1) in head and neck squamous cell carcinoma (HNSCC) and colorectal cancer [33,34], xeroderma pigmentosum complementary group D (XPD), DNA damage binding protein 2 (DDB2), and MMP-9 in lung cancer [35-37].