XIAP and cancer: X-linked inhibitor of apoptosis protein (XIAP), cellular inhibitors of apoptosis 1 and 2 (cIAP 1 and cIAP 2) belong to the inhibitor of apoptosis proteins (IAPs), which represent a family of endogenous caspase inhibitors and also play an important role in inducing apoptosis of cancer cells [33,34,35].