XIAP and head and neck squamous cell carcinoma: Other inhibitors of apoptosis proteins (IAP), including XIAP (X-linked inhibitor of apoptosis), cIAP1/2 (cellular inhibitor of apoptosis 1 and 2), and SMAC (second mitochondria-derived activator of caspases), have also been found to play crucial roles as survival factors in HNSCC [90].