Criterion values for multiplex SNP genotyping by the Invader assay

We have developed a multiplex, single nucleotide polymorphism (SNP) typing system for forensic identification, based on the Invader assay. Using this system, fluorescence data for 21 SNP loci were collected and analyzed. We used endpoint genotyping, commonly used with the Invader assay, and also developed more reliable typing criteria because endpoint typing was occasionally unable to clearly identify the genotype of some loci. Analysis of the fluorescence data identified criteria that had high reproducibility for each genotype. One such criterion is the climbing angle of the curve resulting from two-dimensional plots of the two kinds of fluorescence used for the Invader assay. The climbing angle was observed at the time during the reaction when either or both of the fluorescence intensities increased most significantly. The angles were remarkably associated with homozygous genotypes. On the other hand, all heterozygote endpoint fluorescence ratios were highly reproducible and had very little aberration. These values enabled SNP typing to be more clearly defined compared with typing using only the endpoint fluorescence ratio, which is commonly used with the Invader assay. The values suggested in this study are easily calculated from raw fluorescence data and will be useful for multiplex SNP typing based on the Invader assay.