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Klinefelter syndrome, also known as XXY syndrome, is a fairly common genetic condition found in males only. It occurs when a boy is born with an extra sex chromosome in most or all of his cells. Many boys with Klinefelter syndrome have no signs or symptoms, and some don't even know they have it until later in life. Klinefelter syndrome typically causes a boy's testicles to grow at a slower rate than those of other boys. It also prevents the testicles from producing normal amounts of sperm and the hormone testosterone. Testosterone affects the way a boy develops both physically and sexually. Low hormone levels and problems with sperm production make it difficult or sometimes impossible for a boy with Klinefelter syndrome to father a child later in life.