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Test Catalog

Test ID: FXIST    
X-Inactivation (XIST), Xq13.2 Deletion, FISH

Useful For Suggests clinical disorders or settings where the test may be helpful

As an aid in the diagnosis of Turner syndrome, in conjunction with CMS / Chromosomes Analysis, For Congenital Disorders, Blood


To characterize marker chromosomes that are derived from the X chromosome

Genetics Test Information Provides information that may help with selection of the correct test or proper submission of the test request

Only appropriate to characterize X-derived structurally abnormal chromosomes.

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Turner syndrome is characterized by ovarian hypofunction, short stature, loose skin folds at the back of the neck, and cubitus valgus (elbow deformity) and results from complete or partial monosomy of the X chromosome.


Phenotypic expression of Turner syndrome patients is largely dependent on the patient’s karyotype and identification of sex chromosomes mosaicism plays a key role in clinical management. In mosaicism, 2 or more populations of cells with different karyotypes are present (eg, 45,X/46,XX). Additionally, mental retardation is more common in patients with a small ring chromosome derived from an X chromosome with a deletion of the X-inactivation center (XIST) at Xq13.2.


Fluorescence in situ hybridization (FISH) studies are highly specific and do not exclude other chromosome abnormalities, we recommend that patients suspected of having Turner syndrome also have conventional chromosome studies (CMS / Chromosomes Analysis, for Congenital Disorders, Blood) performed to rule out other chromosome abnormalities or translocations.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

An interpretive report will be provided.

Interpretation Provides information to assist in interpretation of the test results

Any individual with a normal signal pattern (signal on each normal X homolog) in each metaphase is considered negative for a deletion in the region tested by this probe.


Any patient with a FISH signal pattern indicating loss of the XIST critical region will be reported as having a deletion of the regions by this probe.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Because this FISH test is not approved by the FDA, it is important to confirm Turner syndrome diagnoses by other established methods, such as clinical history or physical examination.

Supportive Data

FISH analysis was performed on a series of 33 patient specimens (peripheral blood or amniotic fluid) and results were compared to cytogenetic analyses and the patient's phenotype. This FISH analyses was performed on 24 samples that demonstrated a ring chromosome derived from the X chromosome. In 23 cases, XIST was present on the ring chromosome. In 1 case a deletion of XIST was identified. X chromosome rearrangements were identified in an additional 8 patients, demonstrating this probe’s ability to define other chromosome X rearrangements that are not consistent with XIST. No deletion of XIST was seen in a patient with a normal karyotype.

Clinical Reference Provides recommendations for further in-depth reading of a clinical nature

1. Van Dyke DL, Wiktor A, Palmer CG, et al: Ullrich-Turner syndrome with a small ring X chromosome and presence of mental retardation. Am J Med Genet 1992;43:996-1005

2. Sybert VP, McCauley E: Turner’s syndrome. N Engl J Med 2004;351:1227-1238

Special Instructions and Forms Describes specimen collection and preparation information, test algorithms, and other information pertinent to test. Also includes pertinent information and consent forms to be used when requesting a particular test