Models for Survivin function. Several models are proposed that might explain how Survivin directly or indirectly inhibits caspases. Survivin might directly bind and inhibit caspase, analogous to IAPs, such as XIAP (top). Survivin might sequester SMAC, thus protecting IAPs from this inhibitory protein (middle). Finally, Survivin might somehow enhance the function of IAPs, having a function opposite to SMAC (bottom).