Sequence specific effectiveness of chemically synthesized  antisense oligodeoxynucleotides on hybridization to an accessible site on target mRNA, resistance to degradation  by cellular nucleases, and are delivevered into cells.

Oligonucleotides with about 15-20 nucleotides have sufficient specificity to hybridize unique mRNA.

The most commonly used antisense oligonucleotides has a sulfur group in place of free oxygen of phosphodiester bond the RNA-DNA duplex activates endogenous enzyme  ribonuclease (RNase) H, which cleaves the mRNA component of hybrid molecule.

Clinical trials are undergoing using phosphorothioate  antisense oligonucleotides, against CMV, HIV.