Expressed Sequence Tags (ESTs)
An EST is a short (200-500bp) DNA sequence obtained from one or both ends of a cDNA molecule. Because cDNA is obtained from mRNA, we know that EST sequences are genic. In contrast, sequences obtained from genomic DNA are highly likely to be non-genic. The development of cDNA libraries is technically challenging, but ESTs are highly useful markers. Large numbers of ESTs are publicly available, and primers can easily be designed from these sequences.
Crop |
# of EST sequences |
Maize |
1,464,859 |
Soybean |
1,317,957 |
Rice |
1,220,876 |
Wheat |
1,051,300 |
Barley |
478,734 |
Cowpea |
183,658 |
Cassava |
76,566 |
As of October 3, 2008, NCBI had more than 57,000,000 EST sequences. This figure shows the # available for a few crops. The updated numbers can be found at http://www.ncbi.nlm.nih.gov/dbEST/dbEST_summary.html
An informative tutorial on ESTs can also be found at http://www.ncbi.nlm.nih.gov/About/primer/est.html