Turner syndrome (TS; ORPHA 881) is a rare condition in which all or part of one X chromosome is absent from some or all cells. It affects approximately one in every 1/2500 liveborn girls. The most frequently observed karyotypes are 45,X (40–50%) and the 45,X/46,XX mosaic karyotype (15–25%). Karyotypes with an X isochromosome (45,X/46,isoXq or 45,X/46,isoXp), a Y chromosome, X ring chromosome or deletions of the X chromosome are less frequent. The objective of the French National Diagnosis and Care Protocol (PNDS; Protocole National de Diagnostic et de Soins) is to provide health professionals with information about the optimal management and care for patients, based on a critical literature review and multidisciplinary expert consensus. The PNDS, written by members of the French National Reference Center for Rare Growth and Developmental Endocrine disorders, is available from the French Health Authority website. Turner Syndrome is associated with several phenotypic conditions and a higher risk of comorbidity. The most frequently reported features are growth retardation with short adult stature and gonadal dysgenesis. TS may be associated with various congenital (heart and kidney) or acquired diseases (autoimmune thyroid disease, celiac disease, hearing loss, overweight/obesity, glucose intolerance/type 2 diabetes, dyslipidemia, cardiovascular complications and liver dysfunction). Most of the clinical traits of TS are due to the haploinsufficiency of various genes on the X chromosome, particularly those in the pseudoautosomal regions (PAR 1 and PAR 2), which normally escape the physiological process of X inactivation, although other regions may also be implicated. The management of patients with TS requires collaboration between several healthcare providers. The attending physician, in collaboration with the national care network, will ensure that the patient receives optimal care through regular follow-up and screening. The various elements of this PNDS are designed to provide such support.

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We report the case of monozygotic (MZ) male twin fetuses with different Down syndrome (DS) phenotypes. Prenatal fetal sonography showed a bichorial biamniotic pregnancy with increased nuchal translucency in twin A and a cervical cystic hygroma and heart defect in twin B. Cytogenetic analysis performed after double amniocentesis showed free and homogeneous trisomy 21 in both twins. Monozygosity was confirmed by molecular analysis. The pregnancy was terminated at 17 weeks of gestation (WG). Postmortem analysis confirmed the phenotypic discordance. To our knowledge, this is the first reported prenatal diagnosis of MZ male twins with different Down syndrome phenotypes but identical karyotypes. We discuss the mechanisms involved in phenotypic discordance of monozygotic twins and particularly the role of environmental factors.

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