Klinefelter syndrome (KS) is the leading genetic cause of primary hypogonadism and infertility in men.1,2 The clinical phenotype has expanded beyond the original description of infertility, small testes and gynecomastia.3Animal models, epidemiological studies, and clinical research of males with KS throughout the lifespan have allowed us to better characterize the variable phenotype of this condition. This review will provide an overview on what is known of the epidemiology, clinical features, and pathophysiology of KS, followed by a more focused discussion of testicular development and the clinical management of hypogonadism and fertility in men with KS.

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