Klinefelter syndrome is a genetic condition in which a boy is born with an extra X chromosome. What is Klinefelter syndrome? Klinefelter syndrome, also known as the XXY condition, is a term. Boys who have Klinefelter syndrome are born with it. Causes · One extra copy of the X chromosome in each cell (XXY), the most common cause · An extra X chromosome in some of the cells (mosaic. Klinefelter syndrome is a chromosomal condition in boys and men that can affect physical and intellectual development. Breasts or breast growth (called gynecomastia, pronounced GUY-nuh-kow-mast- ee-uh).