Genotyping of SNP G1822A on 1 ng of genomic DNA in a single-tube reaction. Ligation reactions were performed with 1 ng of Alu I digested genomic DNA in single tubes containing both of the allele-specific OCPs. ERCA reactions included both of the allele-specific P2 Amplifluor primers. Real-time amplification signal was observed only when the OCP/ P2 combination matched the SNP nucleotide on the genomic DNA. Therefore, FAM and TET P2s gave a signal with DNA homozygous for the A and G alleles respectively. DNA heterozygous for the SNP gave an amplification signal with both P2 Amplifluor primers.