Klinefelter Syndrome (KS)

KS describes a set of features that can occur in a male who is born with an extra X chromosome.

The extra X chromosome results from a random error that occurs when a sperm or egg is formed.

Researchers estimate that 1 in about 500-1,000 newborn males has one extra X chromosome. A third or fourth X chromosome in a male is much rarer.

Many factors can affect whether a person with KS has no, mild, or severe symptoms.

Because symptoms can be mild, many people with KS are never diagnosed or treated.

The only way to confirm the presence of an extra chromosome is by a karyotype test.

There is no cure, but many symptoms can be successfully treated.

NICHD conducts and supports research on Klinefelter syndrome and many disorders associated with it.

NICHD conducts and supports a variety of clinical research projects related to Klinefelter syndrome.

Find answers to other common questions about Klinefelter syndrome, such as what other conditions are associated with KS and how KS affects fertility.

Links to websites of groups that study or provide information about Klinefelter syndrome.