text: Many enzymes and proteins play a role in degrading mRNA. For example, in Escherichia coli there are three enzymes: RNase II, PNPase, and RNase R. RNase R is a 3’-5’ exoribonuclease that is recruited to degrade a defective mRNA. RNase R has two structural domains, an N-terminal putative helix-turn-helix (HTH) and a C-terminal lysine(K-rich) domain. These two domains are unique to RNase R, and are attributed as being the determining factors for the selectivity and specificity of the protein.  Evidence has been shown that the K-rich domain is involved in the degradation of non-stop mRNA. These domains are not present in other RNases. Both RNase II and RNase R are members of RNR family, and they share a noteworthy similarity in primary sequence and domain architecture. However, RNase R has the ability to efficiently degrade mRNA, while RNase II has less efficiency in the degrading process. Nevertheless, the specific mechanics of degrading mRNA via RNase R has remained a mystery.