MMP9 and head and neck squamous cell carcinoma: • Tumorigenic phenotype, self-renewal and stemness properties, and resistance to radiotherapy and chemotherapy in HNSCC [49]. • Sphere-forming, tumorigenic, and metastatic abilities in ACC [123]. • Colocalization with Snail and MMP-9 and induction of EMT-related genes in HNSCC [49, 52]. • Strong correlation with nodal metastasis and cisplatin resistance [124, 125].