Named in 1942 for Massachusetts General Hospital physician Dr. Harry Klinefelter, Klinefelter syndrome is a genetic disorder affecting males. People with this syndrome are born with at least one extra X chromosome.

Chromosomes are structures in the nucleus of every cell in the body. Chromosomes contain the genetic information necessary to direct the growth and functioning of all the cells and systems of the body. A normal person has a total of 46 chromosomes in each cell, two of which are responsible for determining that individual's sex. Females have two X chromosomes; males have one X and one Y chromosome.

In Klinefelter syndrome, an error very early in development results in an abnormal number and arrangement of chromosomes. Most commonly, a patient with Klinefelter syndrome will be born with 47 chromosomes in each cell. The extra chromosome is an X chromosome. Some Klinefelter patients have even more complex chromosomal...