Expressed Sequence Tag

Expressed Sequence Tags (EST) are short DNA sequences of mostly 100-800 base pairs in length, which are obtained starting from the 5 'or 3' end by the partial sequencing of cDNAs. Because cDNAs are generated by reverse transcription of mRNA, ESTs thus constitute a section of the sequence of genes that are expressed in the considered living organisms, tissues or cell type, that are active. ESTs are generated in comparison with complete cDNA sequences or genome sequencing with relatively little effort. The sequence information obtained can then be fed to an EST database and used by sequence comparisons and bioinformatic methods as the basis of further analysis. ESTs always go back only to a single sequencing, using different cDNAs differently reproduce well and let sequencing. This and the partially random approach of EST recovery processes cause ESTs sequences represent relatively low reliability. Since ESTs represent only the sequences of mature mRNAs are also introns, promoters and other regulatory elements of genes important not present in EST databases.

Applications for ESTs

• Genome Landmarks: Since ESTs usually represent sequences that occur only once in a genome, they can serve as points of orientation during assembly of the costs incurred in the context of a genome sequencing sequence data. This played, for example, in deciphering the human genome a crucial role.
• Expression analysis: From the occurrence and frequency of ESTs to rough information for the expression of genes can win. This process is sometimes referred to in reference to the Northern blot as a virtual Northern blot.
• Identification of unknown genes: A comparison of ESTs with the sequences of known genes related genes can be identified in the same or other organisms. The ESTs can then serve as a gene probe to the entire sequence of the gene, for example, to win in a cDNA library or by RACE- PCR.