SMAC/Diablo is one of the proapoptotic proteins. SMAC promotes caspase activation in the cytochrome c/Apaf-1/caspase-9 pathway by binding to inhibitor of apoptosis proteins (IAPs) and removing their inhibitory activity. In healthy cells, SMAC is a mitochondrial protein, but when cells undergo apoptosis, SMAC is released into the cytosol. SMAC is ubiquitously expressed with highest expression in testis. Expression is also high in heart, liver, kidney, spleen, prostate and ovary. Low in brain, lung, thymus and peripheral blood leukocytes.