Klinefelter Syndrome is a common chromosomal variation. It is also known as 47,XXY or XXY. On this site we use the terms interchangeably.

The sex chromosomes are the X and Y chromosomes. In most people there are 46 chromosomes, arranged as XY or XX. When a person is born with extra sex chromosomes it is called a sex chromosome variation or anomaly. A person with Klinefelter Syndrome has an extra X chromosome to make up the genetic profile 47,XXY.

Some individuals with Klinefelter Syndrome have a mosaic genetic profile, which means the extra X is in some of the cells in their body but not all.

Other sex chromosome variations include XO, XYY, XXYY, XXX, 48,XXXX and many more!

The predicted incidence of 47,XXY is 1 in 450 to 1 in 600 births (research gives varying information). This makes it one of the most common chromosomal variations. However, it’s estimated that up to 75% of cases are undiagnosed or misdiagnosed.

Klinefelter Syndrome has a wide spectrum of affect. Some individuals are barely or not at all affected by their variant chromosomal makeup, and some men go through their entire lives not realising they have an extra X. Others are more severely affected and will benefit from early intervention as children and ongoing support as adults.

Some effects of Klinefelter Syndrome MAY include

  • Speech and language delays
  • Learning difficulties
  • Emotional, social and behavioural challenges
  • Cognitive processing problems, Auditory Processing Disorder, ADHD
  • Autism Spectrum Disorder traits
  • Low muscle tone and gross and fine motor skills challenges
  • Hormone imbalance and lack of testosterone creating fatigue, increased height, small testicles, infertility, gynecomastia
  • Health risks including anxiety and mood disorders, osteoporosis, cancers


While an individual with Klinefelter Syndrome may have some of these effects, they are very unlikely to have all of them. Most individuals with Klinefelters lead happy and fulfilling lives.