MMP9 and head and neck squamous cell carcinoma: In HNSCC, the abnormal activation of JAK/STAT pathway can also promote the expression of PD‐1 and matrix metalloproteinase 9 (MMP‐9), creating an immunosuppressive state and inducing the degradation of the extracellular matrix (ECM), which promotes the progression and metastasis of HNSCC.