Turner syndrome occurs due to atypical sex chromosome patterns and affects development in females who only have one X chromosome (XO); it occurs in 1 in 2,000 females. The most common feature is short stature, which becomes evident by about age 5. An early loss of ovarian function is also very common, so many affected girls do not undergo puberty unless they receive hormone therapy, and most are unable to conceive. One third to one half of those with Turner syndrome are born with a heart defect. There are also a wide range of other symptoms such as small lower jaw, webbed neck, narrow hips, and misshapen internal organs.