Single-Base Extension or Single Nucleotide Primer Extension

                         Single-base extension (SBE) is a method used for the accurate detection of single-nucleotide polymorphism variants in transcripts. Standard labeled Sanger dideoxynucleotide terminators (ddNTP), which lack the 3' hydroxyl group needed for chain elongation, are used in the extension reaction; as a result, the primer is extended by a single nucleotide. The reaction mixture is subsequently separated by CGE. 

                         For example, Leber’s hereditary optic neuropathy was detected by annealing a primer immediately 5' to the mutation on the template and extending the primer by one fluorescently labeled ddNTP complementary to the mutation. By using two or more differently labeled terminators, both the mutant and wild-type sequences could be detected simultaneously. 

                          In addition, multiplexed SBE genotyping has been applied to the detection of known point mutations on the gene using three unique primers that probe for mutations of clinical importance on this gene and to the diagnosis of multiple endocrine neoplasia type 2 by the analysis of codons 634 and 918 on the RET proto-oncogene.

                          A different field of application is the determination of ovine prion protein allelic variants at codons 136, 154, and 171, where the four mutations responsible for amino acid changes are typed simultaneously.