 With ddNTPs, probes hybridize to the target DNA immediately upstream of SNP nucleotide, and a single, ddNTP complementary to the SNP allele is added to the 3’ end of the probe (the missing 3'-hydroxyl in didioxynucleotide prevents further nucleotides from being added).Each ddNTP is labeled with a different fluorescent signal allowing for the detection of all four alleles in the same reaction.With dNTPs, allele-specific probes have 3’ bases which are complementary to each of the SNP alleles being interrogated.