asRNAs, only few were proven to be actual asRNAs via further functional tests. To minimize the number of false positive results, new approaches from recent years have been focusing on strand-specific transcription, chromatin binding noncoding RNAs and single cell studies. The idea of asRNAs as drug targets started in 1978 when Zamecnik and Stephenson found an antisense oligonucleotide to the viral RNA of Rous scarcoma virus that was capable of inhibiting viral replication and protein synthesis. Since then, much effort has been devoted to developing asRNAs as drug candidates. In 1998, the first asRNA drug, fomivirsen, was approved by FDA. Fomivirsen, a 21 base-pair oligonucleotide, was developed to treat cytomegalovirus retinitis in patients with AIDS. It works by targeting the transcribed mRNA of the virus and consequently inhibiting replication of cytomegalovirus. Despite fomivirsen being discontinued in 2004 due to the loss of the market, it served as a successful and inspiring example of using asRNAs as drug targets or drug candidates. Another example of using an asRNA as a therapeutic agent is mipomersen, which was approved by FDA in 2013. Mipomersen was developed to manage the level of low-density lipoprotein cholesterol (LDL) in patients with homozygous familial hypercholesterolemia (HoFH), which is a rare autosomal dominant genetic condition. Because of the high level of total cholesterol (650–1000 mg/dL) and LDL receptor (above 600 mg/dL) in HoFH, patients with HoFH has a high risk for coronary heart disease. Because the protein apo-B-100 has been found to be required to produce very low-density lipoprotein (VLDL) and LDL, mipomersen complements with the mRNA of apo-B-100 and target it for RNAse H dependent degradation. Ultimately, mipomersen is able to reduce the level of LDL. Examples across species The initial asRNAs discovered were in prokaryotes including plasmids, bacteriophage and bacteria. For example, in plasmid ColE1, the asRNA termed RNA I plays an important role in determining the plasmid copy number by controlling replication. The replication of ColE1 relies on the transcription of a primer RNA named RNA II. Once RNA II is transcribed, it hybridizes to its DNA template and later cleaved by RNase H. In the presence of the asRNA RNA I, RNA I and RNA II forms a duplex which introduces a conformational change of RNA II. Consequently, RNA II cannot hybridize with its DNA template which results in a low copy number of ColE1. In bacteriophage P22, the asRNA sar helps regulate between