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Objective: To localize the different proteins involved in the executioner step of apoptosis in the human testicular tissue: effector caspases (3, 6, and 7), caspase inhibitors called inhibitor of apoptosis proteins (IAPs, such as XIAP), and IAP inhibitors such as Smac/DIABLO and to investigate XIAP and Smac expression and activation of caspase-3 in azoospermia., Design: Retrospective study., Setting: The Biology of Reproduction Center of Poissy and Inserm U407, France., Patient(s): Twenty-five patients diagnosed as azoospermic and 4 patients with normal testicular histology., Intervention(s): Testicular biopsies for histopathological assessment., Main Outcome Measure(s): Localization of proteins by immunohistochemistry., Result(s): Effector caspase-7 seemed to be absent from normal human testes, whereas procaspase-3 and procaspase-6 were detected in both somatic and germ cells. XIAP was mainly expressed in Sertoli cells, whereas its inhibitor Smac was detected in pachytene spermatocytes. On the other hand, although few apoptotic germ cells were detected in biopsies from patients with obstructive azoospermia, increased levels of apoptotic germ cells were detected in spermatogenetic arrest. This increase in apoptotic germ cells was associated with increased levels of active caspase-3 in patients with spermatogenetic arrest, whereas the expression of XIAP and Smac/DIABLO was at similar levels in all groups., Conclusion(s): Active caspase-3 might be important in the apoptotic process observed in spermatogenetic arrest.