An expressed sequence tag (EST) is a segment of sequence from a cDNA clone that corresponds to an mRNA. ESTs are a resource for the annotation of genomic sequences and the analysis of gene expression associated with specific tissues or growth conditions.

EST sequencing is a rapid method to establish an inventory of expressed genes,by determining partial sequences of the cDNAs. Any variety of cDNA libraries may be used for sequencing. Single-pass sequencing is done from one end only of the cDNA clone. This provides a rapid and relatively inexpensive store of data. However, single-pass sequencing means that the sequences obtained contain several errors. Still, sufficient sequence can be obtained from as few as 150 bases for many purposes.

The first undertaking of large-scale partial sequencing of anonymous cDNA clones began with a group of scientists at the National Institutes of Health...