PDCD4 and head and neck squamous cell carcinoma: The literature shows that miR-21 is one of the most studied miRNAs.[116] Although miR-21 functions as an oncogene via programmed cell death 4(PDCD4) in HNSCC,[81] and esophageal carcinoma,[117] it stimulates cell invasion and promotes metastasis in breast cancer by targeting both PDCD4, and tropomyosin 1 (TPM1), a tumor suppressor gene.[118-119] On the other hand, miR-21 controls esophageal squamous cell carcinoma invasion by targeting the tissue inhibitor of matrix metalloproteinase-3 (TIMP3) molecules which control ECM.[120] miRNAs expression pattern and function are cancer-type-specific.