Klinefelter syndrome, a genetic condition affecting males, occurs approximately once in every 650 male births, although estimates vary. According to the National Institutes of Health (NIH), it affects:

Prevalence
· 1 in 650 male births (classic Klinefelter syndrome, 47,XXY)
· 1 in 1,000 male births (including variants, such as 48,XXXY and 49,XXXXY)

Incidence by age
· Newborns: 1 in 1,000 to 1 in 500
· Adults: estimated 3 in 1,000 to 1 in 500

Keep in mind that these numbers may vary depending on the population studied and diagnostic criteria.