Klinefelter syndrome is a group of conditions affecting the health of males who are born with at least one extra X chromosome. Chromosomes, found in all body cells, contain genes. Genes provide specific instructions for body characteristics and functions. For example, some genes determine height and hair color. Other genes influence language skills and reproductive functions. Each person typically has 23 pairs of chromosomes. One of these pairs (sex chromosomes) determines a person’s sex. A baby with two X chromosomes (XX) is female. A baby with one X chromosome and one Y chromosome (XY) is male. Most males with Klinefelter syndrome, also called XXY males, have two X chromosomes instead of one. The extra X usually occurs in all body cells. Sometimes the extra X only occurs in some cells, resulting in a less severe form of the syndrome. Rarely, a more severe form occurs when there are two or more extra X chromosomes.