Fig 1

The methodology used, called allele-specific amplification, uses primers specific to each mutation being tested. When a specimen is heterozygous for a mutation (or a compound heterozygous for two mutations), two peaks are seen. Homozygosity for the wild type allele or a specific mutation reveals only one peak. The results shown here demonstrate one peak for the delta F508 mutation in a CVS (chorionic villus sample) specimen, which is consistent with the fetus being affected with cystic fibrosis (CF). Because the unaffected mother of this fetus also had a similar, seemingly affected, genotype the possibility that a normal allele failed to amplify was raised. This was proven by sequencing the primer-binding sites of the mother and fetus, which revealed a polymorphism that prevented amplification of the wild type allele.