BIRC2 and head and neck squamous cell carcinoma: X-linked IAP (endogenous mitochondrial pathway) and cIAP1/2 (exogenous TNF receptor pathway) are overexpressed in head and neck squamous cell carcinoma (HNSCC) cells, increasing apoptosis resistance and reducing the sensitivity of cancer cells to radiotherapy by blocking downstream caspase activity that is essential for the internal and external apoptotic pathways [34].