Prenatal diagnosis of Down's syndrome by analysis of the chromosome complement of foetal cells obtained by amniocentesis usually takes about 15 days because of the time needed to grow the foetal cells in culture. By using the polymerase chain reaction to amplify specific DNA sequences on chromosome 21, and then measuring the number of copies of these sequences in the foetal cells, the time for diagnosis can be shortened to one day [Verma, L. et al. (1998) Lancet 352, 9-12]. Normal individuals have two copies of chromosome 21 sequences, while Down's syndrome individuals have three. The method is very accurate (no false positive or negative results so far in more than 2000 analyses) and over 99% informative.
Comment: This technique looks very promising. If the authors' estimates of 5000 samples per scientist per year (cf 400 for chromosome analysis) and a total consumable cost of less than £5 per PCR assay are accurate, this may become the method of choice for Down's syndrome diagnosis.