Dideoxynucleotides triphosphates (DdNTPs) are chain-terminating nucleotides used in sequencing techniques (e.g. Sanger Sequencing) to analyze the nucleotide sequence of DNA. Because DdNTPs have a hydrogen molecule (-H) instead of a hydroxyl group (-OH) attached to the 3’-C of its deoxyribose, it cannot bind to any incoming nucleotides. Therefore, addition of DdNTPs during DNA replication can be used to terminate the synthesis reaction.