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A "gene family" is a group of closely related genes that produces similar protein products. Once cDNA representing an expressed gene has been isolated, scientists can then sequence a few hundred nucleotides from either end of the molecule to create two different kinds of ~ESTs. Sequencing only the beginning portion of the cDNA produces what is called a 5' EST. A 5' EST is obtained from the portion of a transcript that usually codes for a protein. These regions tend to be conserved across species and do not change much within a gene family. Sequencing the ending portion of the cDNA molecule produces what is called a 3' EST. Because these ~ESTs are generated from the 3' end of a transcript, they are likely to fall within non-coding, or untranslated regions (~UTRs), and therefore tend to exhibit less cross-species conservation than do coding sequences.
A UTR is that part of a gene that is not translated into protein.
Figure 2. An overview of how ~ESTs are generated.
~ESTs are generated by sequencing cDNA, which itself is synthesized from the mRNA molecules in a cell. The mRNAs in a cell are copies of the genes that are being expressed. mRNA does not contain sequences from the regions between genes, nor from the non-coding introns that are present within many interesting parts of the genome.
Source: ESTs: GENE DISCOVERY MADE EASIER, public domain information from the National Library of Medicine (NLM) Web page