Klinefelter syndrome, also called 47,XXY, is a chromosomal condition that affects development in people who are assigned male at birth. The signs and symptoms of Klinefelter syndrome vary. In some cases, the features are so mild that the condition is not diagnosed until puberty or adulthood. Researchers believe that up to 65 percent of people with Klinefelter syndrome are never diagnosed.\n\nIndividuals with Klinefelter syndrome typically have small testes that produce a reduced amount of testosterone (primary testicular insufficiency). Testosterone is the hormone that directs male sexual development before birth and during puberty. A small percentage of affected individuals are born with undescended testes (cryptorchidism). Without treatment, the shortage of testosterone can lead to delayed or incomplete puberty, breast enlargement (gynecomastia), decreased muscle mass, decreased bone density, a reduced amount of facial and body hair, and fatigue. Klinefelter syndrome can make it difficult for people with this condition to have biological children (a condition called infertility), but up to half of people with Klinefelter syndrome may be able to have children using assisted reproductive technologies.. \n\nThe other physical changes associated with Klinefelter syndrome are usually subtle. Most commonly, affected individuals are taller than average and 2 to 3 inches taller than would be expected for their family. Other features can include curved pinky fingers (fifth finger clinodactyly), flat feet (pes planus), and, less commonly, abnormal fusion of certain bones in the forearm (radioulnar synostosis).\n\nChildren with Klinefelter syndrome may have low muscle tone (hypotonia), difficulty coordinating movements, and mild delays of certain developmental skills, such as rolling over or walking. Affected children have an increased risk of mild delays in speech and language development. People with Klinefelter syndrome tend to have better receptive language skills (the ability to understand speech) than expressive language skills (vocabulary and the production of speech) and may have difficulty communicating and expressing themselves. Affected individuals have an increased risk for learning disabilities, most commonly problems with reading (dyslexia) and written expression. People with Klinefelter syndrome very rarely have intellectual disabilities. \n\nIndividuals with Klinefelter syndrome may have have anxiety, depression, impaired social skills, or behavioral differences, such as emotional immaturity during childhood or difficulty with frustration. Affected individuals also have an increased risk for attention-deficit/hyperactivity disorder (ADHD), though they tend to have problems with attention and distractability rather than hyperactivity. People with Klinefelter syndrome are more likely than those without Klinefelter syndrome to have autism spectrum disorder, which is a developmental disorder that affects communication and social interaction.\n\nPeople with Klinefelter syndrome have an increased risk of developing metabolic syndrome, which is a group of conditions that include high blood glucose levels during prolonged periods without food (fasting), high blood pressure (hypertension), increased belly fat, and high levels of fats (lipids) such as cholesterol and triglycerides in the blood. Compared with unaffected people, adults with Klinefelter syndrome also have an increased risk of developing involuntary trembling (tremors) in their arms or hands, breast cancer (if gynecomastia develops), thinning and weakening of the bones (osteoporosis), and autoimmune disorders such as systemic lupus erythematosus and rheumatoid arthritis. Autoimmune disorders are a large group of conditions that occur when the immune system attacks the body's own tissues and organs. [from MedlinePlus Genetics]
