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Klinefelter Syndrome

A genetic disorder in males caused by having one or more extra X chromosomes. Males with this disorder may have larger than normal breasts, a lack of facial and body hair, a rounded body type, and small testicles.

PubMed Health Glossary
(Source: NIH - National Cancer Institute)

About Klinefelter Syndrome (KS)

The term "Klinefelter (pronounced KLAHYN-fel-ter) syndrome," or KS, describes a set of features that can occur in a male who is born with an extra X chromosome (pronounced KROH-muh-sohm) in his cells. It is named after Dr. Henry Klinefelter, who identified the condition in the 1940s.

Usually, every cell in a male's body, except sperm and red blood cells, contains 46 chromosomes. The 45th and 46th chromosomes—the X and Y chromosomes—are sometimes called "sex chromosomes" because they determine a person's sex. Normally, males have one X and one Y chromosome, making them XY. Males with KS have an extra X chromosome, making them XXY.

KS is sometimes called "47,XXY" (47 refers to total chromosomes) or the "XXY condition." Those with KS are sometimes called "XXY males."...Read more about Klinefelter Syndrome
NIH - National Institute of Child Health and Human Development

What works? Research summarized

Evidence reviews

Lipid Screening in Childhood for Detection of Multifactorial Dyslipidemia: A Systematic Evidence Review for the U.S. Preventive Services Task Force [Internet]

For purposes of this report, multifactorial dyslipidemia refers to dyslipidemias involving elevated total cholesterol (TC) or low-density lipoprotein cholesterol (LDL-C) that are not familial hypercholesterolemia (FH). There is evidence that elevated TC and LDL-C concentrations in childhood, and especially adolescence, are associated with markers of atherosclerosis in young adults.

Acute Heart Failure: Diagnosing and Managing Acute Heart Failure in Adults

The need for this guideline was identified as the NICE guidelines on chronic heart failure were being updated. We recognised at this time that there were important aspects of the diagnosis and management of acute heart failure that were not being addressed by the chronic heart failure guideline, which focussed on long term management rather than the immediate care of someone who is acutely unwell as a result of heart failure. The aim of this guideline is to provide guidance to the NHS on the diagnosis and management of acute heart failure.

Lipid Screening in Childhood and Adolescence for Detection of Familial Hypercholesterolemia: A Systematic Evidence Review for the U.S. Preventive Services Task Force [Internet]

Familial hypercholesterolemia (FH) is an inherited disorder of lipoprotein metabolism characterized by highly elevated total cholesterol (TC) concentrations early in life, independent of environmental influences. Around 1 in 200 to 1 in 500 persons in North America and Europe are estimated to have heterozygous FH. When untreated, FH is associated with a high incidence of premature clinical atherosclerotic cardiovascular disease.

Summaries for consumers

Childhood Extracranial Germ Cell Tumors Treatment (PDQ®): Patient Version

Expert-reviewed information summary about the treatment of childhood extracranial germ cell tumors.

Testicular Cancer Screening (PDQ®): Patient Version

Expert-reviewed information summary about tests used to detect or screen for testicular cancer.

Extragonadal Germ Cell Tumors Treatment (PDQ®): Patient Version

Expert-reviewed information summary about the treatment of extragonadal germ cell tumors.

See all (4)

Terms to know

Genes
A unit of genetic material (DNA) inherited from a parent that controls specific characteristics. Genes carry coded directions a cell uses to make specific proteins that perform specific functions.
Sex Chromosomes
A sex chromosome is a type of chromosome that participates in sex determination. Humans and most other mammals have two sex chromosomes, the X and the Y.
Testicles (Testes)
One of two egg-shaped glands inside the scrotum that produce sperm and male hormones. Also called testicle.
Testosterone
A hormone made mainly in the testes (part of the male reproductive system). It is needed to develop and maintain male sex characteristics, such as facial hair, deep voice, and muscle growth. Testosterone may also be made in the laboratory and is used to treat certain medical conditions.
X Chromosome
A sex chromosome is a type of chromosome that participates in sex determination. Humans and most other mammals have two sex chromosomes, the X and the Y. Females have two X chromosomes in their cells, while males have both X and a Y chromosomes in their cells.

More about Klinefelter Syndrome

Photo of a young adult man

Also called: Klinefelter's syndrome, XXY condition, XXY syndrome, 47,XXY, KS

See Also: Gynecomastia, Infertility

Other terms to know: See all 5
Genes, Sex Chromosomes, Testicles (Testes)

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