Klinefelter syndrome
(sometimes called Klinefelter's, KS or XXY) is where boys and men are born with
an extra X chromosome. Chromosomes are packages of genes found in every cell in the body. Two
types of chromosome, called the sex chromosomes, determine the genetic sex of a
baby. These are named either X or Y.
Usually, a female baby has two X chromosomes (XX) and a male has one X
and one Y (XY). But in Klinefelter syndrome, a boy is born with an extra copy
of the X chromosome (XXY). The X chromosome is not a "female" chromosome and is present
in everyone. The presence of a Y chromosome denotes male sex.