Please explain the use of expressed sequence tags (ESTs) in gene prediction.?

EST is a short sub-sequence of a transcribed protein-coding or non-protein coding DNA sequence.

http://en.wikipedia.org/wiki/Expressed_sequence_tag

Express Sequence Tags

An expressed sequence tag or EST is a short sub-sequence of a transcribed protein-coding or non-protein coding DNA sequence. It was originally intended as a way to identify gene transcripts, but has since been instrumental in gene discovery and sequence determination. The identification of ESTs has proceeded rapidly, with approximately 6 million ESTs now available in computerized databases. An EST is produced by one-shot sequencing of a cloned mRNA (i.e. sequencing several hundred base pairs from both ends of cDNA clones taken from a cDNA library), and the resulting sequence is a relatively low quality fragment whose length is limited by current technology to approximately 500 to 800 nucleotides. Because these clones consist of DNA that is complementary to mRNA, the ESTs represent expressed portions of genes. Often, they are expressed only in certain tissues at certain points in time. ESTs can be mapped to specific chromosome locations using physical mapping techniques, such as radiation hybrid mapping or FISH. Then the investigator can search for sequence similarity between DNA sequences in a region of interest and ESTs known to be located in the same region (and possibly known to be expressed in a tissue that would be consistent with the disease in question). ESTs are also a useful resource for designing probes for DNA microarrays used to determine gene expression.

Some authors use the term "EST" to describe genes for which no further information exists besides the tag.