2',3'-dideoxynucleotide triphosphate (ddNTP)

A nucleotide analogue lacking the 3'-hydroxyl group present in normal dNTPs. When incorporated by DNA polymerase into a growing chain, it prevents further extension because no 3'-OH is available for the next phosphodiester bond. The selectivity of ddNTP incorporation, at every occurrence of a given base, is the mechanism by which Sanger sequencing reads a sequence.