True about small silencing RNAs

in all prokaryotic and eukaryotic organisms,four main classes of RNA molecules exist mRNA, tRNA, rRNA,and small RNAs. Each differs from the others by abundance,size, function, and general stability.Small RNA: Most of these molecules are complexed with proteins to form ribonucleoproteins and are distributed in the nucleus, the cytoplasm, or both. They range in size from 20 to 1000 nucleotides and are present in 100,000 to 1,000,000 copies per cell, collectively representing <=5% of cellular RNA.Two types: 1) Small Nuclear RNA and Large & Small Noncoding Regulatory RNAs: Micro-RNAs (miRNAs), Silencing RNAs (siRNAs),and Long Noncoding RNAs (lncRNAs)siRNAs are frequently used to decrease or "knock-down" specific protein levels ( siRNA homology-directed mRNA degradation) in experimental contexts in the laboratory, an extremely useful and powerful alternative to gene-knockout technology. Indeed, several siRNA-based therapeutic clinical trials are in progress to test the efficacy of these novel molecules as drugs for treating human disease.Reference: Harpers Biochemistry; 30th edition; Chapter 34; Nucleic Acid Structure and Function