3' dNTPs and 2',3' ddNTPs are referred to as chain terminators. When DNA and RNA are made enzymatically in vitro nucleotides are added to the 3' or hydroxyl terminus of the growing chain. Once a chain terminating nucleotide is incorporated no further additions of nucleotides are possible. This can be accomplished by using our 3'-Deoxy, 2',3'-Dideoxy, 3'-Azido or 3'-Amino nucleotide derivatives. Chain terminators have a wide variety of applications including sequencing, studies of enzyme mechanics and therapeutic use.