Klinefelter syndrome (Monarch Disease Ontology term MONDO_0006823) is a sex chromosome disorder caused by the presence of an extra X chromosome in the male karyotype. Affected individuals are infertile and have a small penis and testes. They tend to have tall stature and long legs and may have difficulties with speech and language development. Gynecomastia may be present. Also known as: 47,XXY syndrome, Klinefelter's syndrome, Klinefelter's syndrome, XXY, XXY syndrome, XXY syndrome (Klinefelter syndrome), XXY trisomy.