Multiple Logistic Regression of DNMT3B -149 C/T, -283T/C, and -579G/T , DHFR, SHMT, and TYMS Polymorphisms in Individuals Free of Disease (Control-C) and Head and Neck Squamous Cell Carcinoma (HNSCC) Groups. Here, SHMT1 is linked to head and neck squamous cell carcinoma.