Klinefelter syndrome is a relatively common chromosomal abnormality that occurs in about 1 of 660 live-born males. The findings can be subtle. The 46, X, i (Xq) karyotype is a variant of Turner syndrome (seen only in females), caused by a defective second X chromosome. The 47, XYY karyotype occurs in about 1 in 1000 live-born males and is associated with taller-than-average stature. A person with a mosaic such as 46, XX/47, XX, +21 has milder features of Down syndrome than a person with the more typical 47, XX, +21 karyotype. The 22q11 deletion syndrome is associated with congenital defects affecting the palate, face, and heart and, in some cases, with T cell immunodeficiency.