New blood test for Down syndrome(point of screening was to spot extra chr...)

Conveniently, the cell-free DNA floating in the mother's blood circulation normally exists in short pieces, averaging 169 nucleotides in length. Quake proposed a "shotgun sequencing" strategy where he would use the very latest high-throughput gene sequencing technology and equally powerful computers to identify millions of unique sequence "tags" in the fetal DNA. Each of these tags was a 25 base-pair fragment of DNA. After the tags were identified, Quake's group could then map them to specific locations on the 23 pairs of chromosomes. By using this strategy, the researchers believed they could detect higher-than normal amounts of DNA belonging to the three chromosomes involved in the most common aneuploidies -- chromosomes 21, 18, and 13.

In the experiments reported in PNAS, Quake and his colleagues, including first author H. Christina Fan, a graduate student in bioengineering at Stanford, used their new technique to analyze DNA in blood samples from 18 pregnant women who were undergoing invasive prenatal testing (and one man, for reference).

When the amounts of DNA corresponding to each chromosome were plotted on a graph, significant, above-normal peaks appeared when a chromosome was present in three copies instead of two. These signals enabled the scientists to correctly identify the 12 women who carried aneuploid fetuses and the six whose pregnancies were normal. Their results matched those of amniocentesis or CVS test that were done on the women.

Quake said the shotgun-sequencing system is not only safer than invasive techniques but can also make a determination at an earlier gestational age, around 12 weeks.

The earlier the couple knows the status of the pregnancy,
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