lytic and lysogenic cycle by control the expression of Ant. Besides being expressed in prokaryotes, asRNAs were also discovered in plants. The most well described example of asRNA regulation in plants is on Flowering Locus C (FLC) gene. FLC gene in Arabidopsis thaliana encodes for a transcription factor that prevent expression of a range of genes that induce floral transition. In cold environment, the asRNA of FLC gene, denoted COOLAIR, is expressed and inhibits the expression of FLC via chromatin modification which consequently allows for flowering. In mammalian cells, a typical example of asRNA regulation is X chromosome inactivation. Xist, an asRNA, can recruit polycomb repressive complex 2 (PRC2) which results in heterochromatinization of the X chromosome. Classification Antisense RNAs can be classified in different ways. In terms of regulatory mechanisms, some authors group asRNAs into RNA-DNA interactions, RNA-RNA interactions either in nucleus or cytoplasm and RNA-protein interactions (epigenetic). Antisense RNAs can be categorized by the type of the promoters that initiate expression of asRNAs: independent promoters, shared bidirectional promoters or cryptic promoters. In terms of length, although asRNA in general is classified as lncRNAs, there are short asRNAs with length of less than 200 base pairs. Because the regulatory mechanism of asRNAs are found to be species specific, asRNAs can also be classified by species. One of the common ways of classifying asRNAs is by where the asRNAs are transcribe relatively to their target genes: cis-acting and trans-acting. Cis-acting Cis-acting asRNAs are transcribed from the opposite strand of the target gene at the target gene locus. They often show high degree or complete complementarity with the target gene. If the cis-acting asRNA regulates gene expression by targeting mRNA, it can only target individual mRNA. Upon interactions with the targeting mRNAs, cis-acting asRNAs can either block ribosome binding or recruit RNAase to degrade the targeting mRNAs. Consequently, the function of these cis-acting asRNAs is to repress translation of the targeting mRNAs. Besides cis-acting asRNAs that target mRNAs, there are cis-acting epigenetic silencers and activators. Antisense RNA has been shown to repress the translation of LINE1-ORF2 domain of Entamoeba histolytica. However it is not confirmed yet whether its cis-acting or trans. In terms of epigenetic modification, cis-acting refers to the nature of these asRNAs that regulate epigenetic changes around the loci where they are transcribed. Instead of targeting individual mRNAs, these cis-acting epigenetic regulators can recruit chromatin modifying enzymes which can