Expressed Sequence Tags (ESTs)

    In vivo, the exon regions of eukaryotic genes are transcribed as part of the heterogeneous nuclear RNAs (hnRNAs) and are subsequently spliced together to form mRNAs. In vitro, reversal of this process is possible by reverse transcription of the complete set of mRNAs present in any tissue. This yields a library of complementary DNAs (cDNAs) that correspond to the genes expressed in that sample. Sequencing a short segment (<100 bases) at one or both ends of each cDNA produces a "tag" with enough information to identify the expressed gene: these are called expressed sequence tags (ESTs).  Comparison of the ESTs from a novel genome or tissue type with those from a genome that is well-characterized allows identification of the function of gene loci expressed in the new organism.

Figure modified after Griffiths et al. 2002; text material ©2015 by Steven M. Carr