Objective:Non-invasive prenatal diagnosis of chromosome aneuploidies has been achieved by measuring the ratio of two alleles of a single nucleotide polymorphism (SNP) in circulating placental mRNA (the RNA–SNP allelic ratio approach) in maternal plasma. This research is conducted to develop a rapid,accurate and cost-effective assay for the non-invasive prenatal detection of fetal trisomy 21. Methods:Maternal plasma samples were collected from 50 eupliod pregnancies(gestational age range:15 to 21weeks)and 13 trisomy 21 pregnant cases(gestational age range:19 to 26 weeks). With the application of Multiple-SNaPshot assay in measuring five SNP allelic ratios and heterozygosis for the chromosome 21 transcribed placenta-specific-4(PLAC4) mRNA in maternal plasma,we successfully achieved noninvasive prenatal detection of trisomy 21 in cases with the heterozygous SNPs on PLAC4 mRNA. Also the genotypes of the SNPs located in the transcribed regions of the gene PLAC4 were detected in population samples collected from 200 voluteers in Southeast China by SNaPshot assay. Results:PLAC4 mRNA was detected in peripheral blood of all pregnant women. Of all 63 singleton pregnancies,17 gravidas were found at least one heterozygous SNP on PLAC4 mRNA in maternal plasma. Among them,3 pregnancies with a trisomy-21 fetus and 14 pregnancies with an euploid fetus were detected by the analysis of the RNA-SNP allelic ratio. The highest heterozygosity frequencies of five SNPs on gene PLAC4 were Rs8130833 and Rs4818219,which were estimated to be 0.278,0.343,respectively. Conclusion:The plasma placental RNA allelic ratio can be used for noninvasive prenatal detection of Downs Syndrome,if SNPs on PLAC4 mRNA in maternal plasma are heterozygous. The two SNPs with higher heterozygosity frequencies on gene PLAC4 were Rs8130833 and Rs4818219 in population of Southeast China.