BIRC2 and head and neck squamous cell carcinoma: Nevertheless, mutations affecting XIAP or caspase-9 occur rarely in HNSCC compared to cIAP1/2 [60, 61] and extrinsic apoptosis effectors such as FADD and caspase-8, suggesting that XIAP and the intrinsic pathway regulated by it are not as important in HNSCC.