EST: Expressed Sequence Tags
The genome of a eukaryotic organism has many genes but each type of cell expresses only a fraction depending on the functions served. As has been seen earlier (Profile 15), a cDNA library obtained from a particular tissue provides a "snapshot" of the genes expressed in that tissue at the time of extraction of mRNAs. Theoretically, the thousands of phage or plasmid clones of the library cover almost all the sequences expressed in the form of mRNA. The strongly expressed genes are represented more than weakly expressed genes (rare mRNAs).