RT Book, Section A1 Backeljauw, Philippe F. A1 Sarafoglou, Kyriakie A1 Stratakis, Constantine A. A2 Sarafoglou, Kyriakie A2 Hoffmann, Georg F. A2 Roth, Karl S. SR Print(0) ID 1140320106 T1 Turner Syndrome T2 Pediatric Endocrinology and Inborn Errors of Metabolism, 2e YR 2017 FD 2017 PB McGraw-Hill Education PP New York, NY SN 9780071773140 LK accesspediatrics.mhmedical.com/content.aspx?aid=1140320106 RD 2024/03/28 AB Turner syndrome (TS) is the most common genetic disorder affecting only females, and occurs in approximately 1 in 2500 female live births.1 The criteria for diagnosis include complete or partial absence of the second sex (X) chromosome (with/without cell line mosaicism) plus short stature and primary ovarian failure with/without the presence of other phenotypic TS features. The genotype in TS as tested in peripheral blood is most commonly 45,X.1 A TS patient with mosaicism, on the other hand, will have two or more cell lines with different chromosome make-up. Thus, apart from 45,X monosomy, other karyotype abnormalities in TS patients include 45,X/46,XX (the most frequent form of mosaicism which often results in a milder clinical phenotype) and 45,X/46,XY (resulting in partial or mixed gonadal dysgenesis). Structural abnormalities of the X chromosome include deletions, rings (rX), and translocations. An isochromosome X (isoXq) is the most common structural chromosome X abnormality. It is an abnormal X chromosome caused by a transverse division of the centromere instead of the normal longitudinal division. The result is two copies of the long arm connected by the centromere, with possibly certain Xp sequences inserted. The most common occurrence of the isoXq is as part of 45,X/46,X, i(X)q mosaicism, because isoX is frequently lost during cell division.1 Overall, mosaicism is found in approximately 80% of patients with 45,X when two or more tissues (eg, lymphocytes and fibroblasts) are examined.2