The most common of the SCAs is 47,XXY, found in as many as 1/600 males [Nielsen and Wohlert, 1990; Ratcliffe, 1994]. The associated phenotypic characteristics were first described by Klinefelter et al. [1942], although the degree to which 47,XXY individuals exhibit these features can vary widely. One of the most common findings in 47,XXY is hypogonadic hypogonadism. For reasons that are not yet well understood, the testicles of individuals with 47,XXY begin to accumulate scar tissue early in life, a process that accelerates during puberty, leaving the vast majority of affected males infertile. Testosterone levels in adulthood tend to be low or within low range of normal and are likely related to several of the characteristic elements of Klinefelter’s, such as eunuchoid body habitus, low energy, metabolic abnormalities, and osteoporosis. The degree to which the prenatal and postnatal testosterone surges are affected is more controversial, with some studies reporting decreased testosterone levels during this period, whereas others do not [Ross et al., 2005; Aksglaede et al., 2006, 2007a; Bastida et al., 2007].
