Klinefelter syndrome (KS), also known as 47,XXY, is a chromosome anomaly where a male has an extra X chromosome. The complications commonly include infertility and small, poorly functioning testicles (if present). These symptoms are often noticed only at puberty, although this is one of the most common chromosomal disorders. The birth prevalence of KS in the State of Victoria, Australia was estimated to be 223 per 100,000 males. It is named after American endocrinologist Harry Klinefelter, who identified the condition in the 1940s, along with his colleagues at Massachusetts General Hospital.
The syndrome is defined by the presence of at least one extra X chromosome in addition to a Y chromosome, yielding a total of 47 or more chromosomes rather than the usual 46. Klinefelter syndrome occurs randomly. The second X chromosome comes from the father and mother nearly equally. An older mother may have a slightly increased risk of a child with KS. The syndrome is diagnosed by the genetic test known as karyotyping.