A genetically inherited sex chromosome abnormality only affecting males. Genotypically, they are XXY or more rarely XXXY, or XXXXY. They have asexual to feminine body contours as well as breast enlargement and relatively little body hair. They are sterile or nearly so and their penis, testes, and prostate gland are small. Like metafemales, most Klinefelter syndrome men are an inch or so above average height. They usually have slight learning difficulties, especially with language. However, most are sufficiently ordinary in appearance and mental ability to live in society without notice. The frequency of Klinefelter syndrome has been reported to be between 1 in 500 and 1 in 1,000 male births. Males with Down syndrome sometimes also inherit Klinefelter syndrome.