Turner syndrome or Ullrich–Turner syndrome (also known as “Gonadal dysgenesis”), 45,X, encompasses several conditions in human females, of which monosomy X (absence of an entire sex chromosome, the Barr body) is most common. It is a chromosomal abnormality in which all or part of one of the X chromosomes is absent or has other abnormalities. It is called 45,X because affected individuals have 45 chromosomes as opposed to the usual 46. In some cases, the chromosome is missing in some cells but not others, a condition referred to as mosaicism or “Turner mosaicism”. Occurring in 1 in 2000 – 1 in 5000 phenotypic females, the syndrome manifests itself in a number of ways. There are characteristic physical abnormalities which affect many but not all people with Turner syndrome, such as short stature, swelling, broad chest, low hairline, low-set ears, and webbed necks. Girls with Turner syndrome typically experience gonadal dysfunction (non-working ovaries), which results in amenorrhea (absence of menstrual cycle) and sterility. Concurrent health concerns may also be present, including congenital heart disease, hypothyroidism (reduced hormone secretion by the thyroid), diabetes, vision problems, hearing concerns, and many autoimmune diseases. Finally, a specific pattern of cognitive deficits is often observed, with particular difficulties in visuospatial, mathematical, and memory areas. Turner syndrome is named after Henry Turner, the endocrinologist who first described it in 1938.