esTs

Expressed sequence tags (ESTs) are short sub-sequences derived from complementary DNA (cDNA) representing portions of expressed genes. They are obtained by sequencing either end of a cDNA clone generated from mRNA, providing a snapshot of genes that are actively transcribed in a given tissue or condition. ESTs were instrumental in large-scale gene discovery and transcript mapping during the 1990s and early 2000s, particularly for organisms with incomplete or no reference genomes.

ESTs are used to identify gene transcripts, help annotate genomes, and facilitate the construction of transcript catalogs. They are integrated into public databases such as dbEST (the EST division of NCBI's dbGSS) and have been clustered and assembled into contigs and tentative consensus sequences to represent unique transcripts. The 5' and 3' ends provide information about transcript structure and potential alternative splicing.

Technically, ESTs are typically 200-800 base pairs long and originate from single-pass sequencing, making them prone to base-call errors and biases. They may contain vector or adaptor sequences, untranslated regions, or chimeric joins. Clustering diverse ESTs helps identify redundancies and generate longer consensus sequences, but EST data can be biased toward highly expressed transcripts and tissue-specific expression.

Despite the rise of high-throughput RNA sequencing, EST resources remain valuable for non-model organisms, historical gene discovery, and cross-species comparisons. They continue to support genome annotation, comparative genomics, and gene-family studies, though newer sequencing methods have largely supplanted routine EST projects.