of this group comprise the largest fraction of the non-coding transcriptome. Short non-coding RNA includes the following members: transfer RNA (tRNA) micro RNA (miRNA): 19-24 nucleotides (nt) long. Micro RNAs up- or downregulate expression levels of mRNAs by the process of RNA interference at the post-transcriptional level. small interfering RNA (siRNA): 20-24 nt small nucleolar RNA (snoRNA) Piwi-interacting RNA (piRNA): 24-31 nt. They interact with Piwi proteins of the Argonaute family and have a function in targeting and cleaving transposons. enhancer RNA (eRNA) Scope of study In the human genome, about 5% of all genes get transcribed into RNA. The transcriptome consists of coding mRNA which comprise around 1-4% of its entirety and non-coding RNAs which comprise the rest of the genome and do not give rise to proteins. The number of non-protein-coding sequences increases in more complex organisms. Several factors render the content of the transcriptome difficult to establish. These include alternative splicing, RNA editing and alternative transcription among others. Additionally, transcriptome techniques are capable of capturing transcription occurring in a sample at a specific time point, although the content of the transcriptome can change during differentiation. The main aims of transcriptomics are the following: "catalogue all species of transcript, including mRNAs, non-coding RNAs and small RNAs; to determine the transcriptional structure of genes, in terms of their start sites, 5′ and 3′ ends, splicing patterns and other post-transcriptional modifications; and to quantify the changing expression levels of each transcript during development and under different conditions". The term can be applied to the total set of transcripts in a given organism, or to the specific subset of transcripts present in a particular cell type. Unlike the genome, which is roughly fixed for a given cell line (excluding mutations), the transcriptome can vary with external environmental conditions. Because it includes all mRNA transcripts in the cell, the transcriptome reflects the genes that are being actively expressed at any given time, with the exception of mRNA degradation phenomena such as transcriptional attenuation. The study of transcriptomics, (which includes expression profiling, splice variant analysis etc.), examines the expression level of RNAs in a given cell population, often focusing on mRNA, but sometimes including others such as tRNAs and sRNAs. Methods of construction Transcriptomics is the quantitative science that encompasses the assignment of a list of strings ("reads") to the object ("transcripts" in the genome). To calculate the expression strength, the density of