Klinefelter Syndrome (KS)

KS describes a set of physical, language, and social development symptoms in males who have an extra X chromosome. Its main feature is infertility. Outward signs of KS can be subtle, so symptoms often are not recognized, and may not be treated in a timely manner. The NICHD is one of many federal agencies and NIH Institutes working to understand KS, discover why it occurs, and identify and treat its symptoms.

Common Name

  • Klinefelter syndrome

Medical or Scientific Names

  • Klinefelter syndrome 
  • 47,XXY 
  • XXY syndrome or condition
  • XXY trisomy
  • 47,XXY/46,XY or mosaic syndrome (rare variation)
  • Poly-X Klinefelter syndrome, including the following rare variations: 
    • 48,XXYY (or tetrasomy)
    • 48,XXXY (or tetrasomy)
    • 49,XXXXY (or pentasomy)
Alternate Titles
XXY Syndrome
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