Down's syndrome results from the production of three copies of chromosome 21 
within a cell. We have devised a method termed the homologous gene quantitative 
polymerase chain reaction (HGQ-PCR), which uses one pair of primers and which 
can directly identify the additional copy of chromosome 21 by simultaneously 
amplifying two highly homologous genes of the human liver-type 
phosphofructokinase located on chromosome 21 (PFKL-CH21) and the human 
muscle-type phosphofructokinase located on chromosome 1 (PFKM-CH1) for 
self-detecting determination. On analysis of 34 cases of Down's syndrome, 
including two cases of unbalanced translocation 46, XY, der (14; 21) (q10; q10), 
+ 21, and 100 normal individuals, the relative ratio of the PFKM-CH1/PFKL-CH21 
product was 1.33 +/- 0.323 (mean +/- SD) and 0.40 +/- 0.16 (mean +/- SD) for 
disomy DNA and trisomy DNA, respectively. The difference between these two 
groups was highly significant (P < 0.001). These results indicate that this 
quantitative method is practical and may be used for the prenatal diagnosis of 
Down's syndrome caused by trisomy 21.