Klinefelter syndrome is a chromosomal condition in boys and men that can affect physical and intellectual development. Most commonly, affected individuals are taller than average are unable to father biological children infertile ; however the signs and symptoms of Klinefelter syndrome vary among boys and men with this condition. In some cases, the features of the condition are so mild that the condition is not diagnosed until puberty or adulthood, and researchers believe that up to 75 percent of affected men and boys are never diagnosed. Boys and men with Klinefelter syndrome typically have small testes that produce a reduced amount of testosterone primary testicular insufficiency. Testosterone is the hormone that directs male sexual development before birth and during puberty. Without treatment, the shortage of testosterone can lead to delayed or incomplete puberty, breast enlargement gynecomastia , decreased muscle mass, decreased bone density, and a reduced amount of facial and body hair.

The term Klinefelter syndrome KS describes a group of chromosomal disorder in which there is at least one extra X chromosome to a normal male karyotype, 46,XY. XXY aneuploidy is the most common disorder of sex chromosomes in humans, with prevalence of one in males. Other sex chromosomal aneuploidies have also been described, although they are much less frequent, with 48,XXYY and 48,XXXY being present in 1 per 17, to 1 per 50, male births.

Sex chromosome abnormalities may be caused by full or partial deletions or duplications of sex chromosomes. Chromosomes are structures within cells that contain DNA and many genes. A gene is a segment of deoxyribonucleic acid DNA and contains the code for a specific protein that functions in one or more types of cells in the body see Genes and Chromosomes for a discussion about genetics.