['To study the expression and localization of Aven in rat and human testis from azoospermic patients with different etiologies and its regulation by estrogens.', 'Experimental study.', 'University research center and private IVF clinic.', 'Six men with obstructive azoospermia, five with hypospermatogenesis, and six with Sertoli cell-only syndrome; male Wistar rats.', 'Testicular biopsies and rat seminiferous tubules (SeT) cultured in the presence or absence of 17β-estradiol (E(2)).', 'Testicular cell localization of Aven protein was analyzed by immunohistochemistry. Expression levels of Aven in testicular biopsies and cultured SeT, in the presence or absence of 17β-estradiol, were determined by quantitative reverse transcription-polymerase chain reaction and Western blot.', 'Aven is expressed in Sertoli cells, spermatocytes, and spermatogonia of both rat and human testis. Aven is underexpressed in the testis of men with nonobstructive azoospermia, and its expression levels correlate with severity of spermatogenic status. Aven expression is regulated by E(2) in rat SeT cultured ex\xa0vivo.'] Is apoptosis-inhibitor Aven downregulated in defective spermatogenesis and a novel estrogen target gene in mammalian testis?