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  Rapid DNA test for the prenatal detection of Down Syndrome

Down syndrome is a non-hereditary congenital disease that is caused by of an extra copy of chromosome 21 (trisomy 21). Trisomy 21 is found in up to 95 percent of all Down syndrome cases, which is the major cause of mental retardation in about one in 800 newborns. Studies indicate that trisomy 21 is more frequent in the fetus of pregnant women over 35 years of age.

Two other common conditions that are due to the presence or absence of extra chromosomes are Klinefelter and Turner syndromes.
Klinefelter syndrome is a genetic abnormality that is caused by an extra X chromosome, and is typically identified as XXY. Klinefelter syndrome occurs in about one of over 600 male births. Turner syndrome results from the loss of one chromosome X. Typically identified as XO, Turner syndrome occurs in about one of every 2,500 female births.

Benefits of FISH Technology

The desire for a healthy child has increased the demand for fetal karyotyping. Fetal karyotyping, which is the traditional method for detecting chromosomal abnormalities, is an expensive and time-consuming process that requires cell culturing and isolation of metaphase cells ( cells that have divided). The diagnosis from fetal karyotyping may be available in two to three weeks.

FISH allows highly sensitive and direct visualization of specific chromosomes, detect and quantifiy the chromosome copy number using fluorescent-labeled probes - segments of labeled DNA produced in the laboratory that binds, or "hybridizes", with target regents of these chromosomes.

Because of the important and proven advantages, FISH Technology, which was until now a powerful research tool, evolved to a very useful clinical diagnostic tool.

This acceptance of FISH as a clinical tool is assisting obstetricians and gynecologists in managing their patients by providing rapid, definitive test results that enable timely decisions regarding patient treatment and care.

FISH has excellent accuracy and reproducibility. It can provide rapid results, that are especially important if the amniocentesis is done late in pregnancy. It also eases parental anxiety, especially when sonographic markers show an increased risk for a chromosomal abnormality. In addition, mosaics of sex chromosomes that were not diagnosed by karyotyping, can be detected.

Also, FISH is of great importance to IVF.
A large proportion of patients undergoing in-vitro fertilization (IVF) are aged 35 years or more. In this age group, about 50 % of embryos are chromosomally abnormal, with aneuploidy being the most common anomaly. As the origin of most aneuploidies is maternal meiosis I non-disjunction, unfertilized oocytes or embryos could be screened for aneuploidy by analysing their first polar bodies or a blastomere (Preimplantation diagnosis). Single cells can be analyzed by fluorescence in-situ hybridization (FISH) using probes simultaneously for chromosomes X,Y,18 AND 13/21.
Preimplantation genetic diagnosis successfully detected these abnormalities.

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