About one in 600 newborn boys will have the genetic condition Klinefelter syndrome (sometimes called Klinefelter's or XXY), which means they are born with an extra X chromosome.
This will have happened randomly during the formation of the egg or sperm, or after the baby boy was conceived. The extra chromosome is not inherited.
Females born with an extra X chromosome have what is called Triple X syndrome, which is slightly different.
Adult males with Klinefelter syndrome may not be able to produce enough testosterone, the sex hormone responsible for the development of male characteristics and important for maintaining bone strength, libido and fertility in men.
The features that result from this – such as low energy and sex drive, lack of muscle and too much body fat around the middle – may not be noticeable until after the age of puberty.
Most men with Klinefelter syndrome live independently and have normal lives, jobs and relationships, and will be unaware of their chromosome variation. Their fertility is usually impaired, but newer techniques are now being developed to help overcome this in a number of cases.
Klinefelter syndrome is hard to spot in infants, as some of the more common features may not be present or obvious at first.
Babies and toddlers with Klinefelter syndrome may:
be born with undescended testicles
be slow to sit up, crawl and walk
not start talking until later than average
have low muscle power
be quiet and passive
Boys and teenagers
Some boys with Klinefelter syndrome can be shy with low self-confidence. About two-thirds may have problems at school with reading, writing, spelling and paying attention. They may also have:
low energy levels
mild learning disabilities such as dyspraxia and dyslexia
difficulty socialising or expressing their feelings
Boys may grow more quickly than usual during childhood and become taller than expected for the family. Most of the extra growth is in the legs, and the hips may get broader too. Extreme tallness is unusual and can be predicted early on in childhood.
Puberty starts at the normal time (11-13 years) with testicle growth as in all boys, but the testicle size does not increase as usual.
The small testicles may not produce enough of the sex hormone testosterone, which may explain some of the physical features, such as:
a flabby body with low muscle tone
a reduced amount of facial and body hair
small, firm testicles and a small penis
enlarged breasts (gynaecomastia; affects about two-thirds)
reduced calcium in the bones
The lack of testosterone may delay the completion of sexual development during puberty.
Even at this later life stage, these signs of Klinefelter syndrome may not be noticeable and the condition may not be diagnosed until adulthood, when the GP is consulted because of fertility problems.
Advice at this stage
See your GP if your son has slow development for his age, especially if you recognise some of the signs listed above.
It's worth diagnosing any sort of growth or developmental problem early, as treatment can prevent or improve some of the outcomes. See What treatments are available? for more information.
Participation in sport will help build muscle strength and independence.
You might find it helpful to connect with other parents of children with Klinefelter syndrome.
Men with Klinefelter syndrome may produce lower amounts of testosterone than usual and will not generally be able to conceive naturally as the testicles produce little or no sperm.
New techniques are being developed to extract sperm from the testicles to help with artificial conception methods, which may help in some circumstances.
If a man with Klinefelter syndrome was not diagnosed and treated at the time of puberty, he may have some or all of the following features:
a low sex drive
taller than average for the family
reduced amounts of facial and body hair
belly fat and looser muscles
enlarged breasts (gynaecomastia)
small testicles and sometimes also a small adult-sized penis
Anxiety, learning difficulties and depression are common, although intelligence is usually unaffected.
Despite these problems, most men with Klinefelter syndrome will have normal independent lives, holding down relationships and jobs.
It may be helpful to talk to your doctor about treatments and discuss family planning options. The useful links on this page list support groups, where links to other men with Klinefelter syndrome can be made.
Men with Klinefelter syndrome are generally healthy, but at slightly greater risk than average of:
varicose veins and other blood vessel problems
autoimmune disorders (where the immune system attacks the body's own tissues) such as type 1 diabetes and lupus
What's the cause?
A male with Klinefelter syndrome is born with an extra X sex chromosome in his cells – XXY, rather than the usual XY (see the box on this page for an explanation of sex chromosomes).
This second X chromosome carries extra copies of genes, which interferes with the development of the testicles.
Some males will have a milder form of Klinefelter syndrome, where they only have XXY chromosomes in some of their cells, rather than all of them. This is known as mosaic Klinefelter syndrome.
How is Klinefelter syndrome diagnosed?
GPs may suspect Klinefelter syndrome after physically examining their patient's genitals and chest and testing a blood or urine sample for the presence of abnormal hormone levels.
The diagnosis can be confirmed after ordering a genetic test, which involves sending a blood sample to a laboratory to check if there is an extra X chromosome (chromosomes can be seen under a powerful microscope).
What treatments are available?
Klinefelter syndrome cannot be cured as there's no way to fix the extra chromosome, but treatments and therapy can help, including:
testosterone replacement therapy
speech and language therapy
educational support (through educational psychology)
emotional and behavioural support (through psychology)
counselling to work through emotional issues and infertility
infertility treatment (IVF)
surgery to remove excess breast tissue
Testosterone replacement therapy (TRT)
TRT involves being prescribed a medication containing a hormone similar to testosterone.
It can be taken in the form of gels or patches, or given as injections to correct the hormone deficiency.
It cannot reverse infertility, but can treat and prevent many of the typical features of Klinefelter syndrome.
Ideally, an assessment should be made at the time of normal puberty, around the age of 12. Even if it is not needed, a repeat assessment should be made in the mid- to late-teens, as a deficiency may show up only at that stage onwards.
If needed, TRT helps with normal development during puberty, such as a deep voice, facial and body hair, and an increase in muscle mass (although it won't cause the testicles to grow). It should also improve bone density and reduce the risk of fractures.
Referral to a paediatric or adult endocrinologist (specialist in hormone disorders) according to age is necessary. The specialist can make the appropriate assessments.
Male and female sex chromosomes
Your sex is determined by genes on the sex chromosomes.
A baby inherits two sex chromosomes – an X chromosome from their mother and either an X or a Y chromosome from their father.
Usually, a female baby has two X chromosomes (XX) and a male has one X and one Y (XY).
A male with Klinefelter has one extra copy of the X chromosome (XXY), but is fully male because of the presence of the Y chromosome.