SiRNAs are antisense oligonucleotides (ASOs), routinely used as gene silencing reagents through the interference of mRNA. Their stability and delivery are the longstanding problems for their clinical applications. To overcome the problems, several backbones modified siRNAs are developed in which phosphorodiamidate morpholino oligonucleotides (PMOs) have become a promising alternative as their drugs (eteplirsen, golodirsen, viltolarsen, and casimersen) have been approved by the US FDA for the treatment of Duchenne Muscular Dystrophy (DMD) and is the only treatment available in the market right now. PMOs are neutral and provide good target predictability, high in-cell efficacy and excellent sequence specificity and their record of safety, efficacy and bioavailability suggest prospective clinical applications. In this context, chemistry and biology of morpholino oligonucleotides (MOs) will be discussed to overcome the problems associated with poor stability of phosphodiester bonds and poor bioavailability of RNA to become a candidate for therapeutics. The synthesis of PMOs remained a significant challenge until we recently developed and disclosed a phosphoramidite chemistry-based method compatible with automated oligonucleotide synthesizers. This approach has been optimized to enable the efficient synthesis of a wide range of PMO–DNA/RNA chimeras for biological screening. Furthermore, additional modifications to the PMO backbone have been introduced to enhance cellular uptake and gene-silencing efficacy, eliminating the need for transfection reagents or other delivery agents. In summary, chemical modification is key to improving WatsonCrick base pairing affinity, metabolic stability, and cellular uptake of RNA and is absolutely essential in the discovery and development of RNA-based therapeutics. Technology transferred to a startup company www.GenElixir.com