ESTs

Expressed Sequence Tags (ESTs) are short (usually between 300-500 nucleotides) DNA sequences generated from one or both ends of cDNA sequences.

 

Because ESTs are generated from cDNA, which was synthesized from mRNA, they are highly likely to be genic.

Sequencing only the leading part of the cDNA produces a 5' EST. This part of the transcript usually encodes for a protein, so this is the type most widely used. Orthologous sequences tend to be conserved across species.

Sequencing the tail end of the cDNA molecule produces a 3'
EST. This end of the transcript often includes a stretch of untranslated message (the 3' UTR), and therefore the sequence tends to be less conserved both within and across species.