This review describes the application of a natural defense mechanism to develop effective agents for The post-transcriptional control of gene expression. 2-5A is a unique 2,5-phosphodiester bond linked oligoadenylate, (pp)p5A2(p5A)n , that is elaborated in virus-infected interferon-treated cells. The 2-5A System is an RNA degradation pathway that is an important mechanistic component of interferons action against certain viruses. It may also play a role in the anticellular effects of interferon and in general RNA decay. A major player in the 2-5A-system is the latent and constitutive 2-5A-dependent ribonuclease (RNase L) which upon activation by 2-5A, degrades RNA. This RNase L enzyme can be recruited for antisense therapeutics by linking it to an appropriate oligonucleotide targeted to a chosen RNA. Syntheses of 2-5A, its analogues, 2-5A-antisense, and its modifications are detailed herein. Applications of 2-5A-antisense to particular targets such as HIV, PKR, chronic myelogenous leukemia, telomerase, and respiratory syncytical Virus is described.