<p>In prokaryotes, the incorporation of selenocysteine as the 21st amino acid, encoded by TGA, requires several elements: SelC is the tRNA itself, SelD acts as a donor of reduced selenium, SelA modifies a serine residue on SelC into selenocysteine, and SelB is a selenocysteine-specific translation elongation factor. 3' or 5' non-coding elements of mRNA have been found as probable structures for directing selenocysteine incorporation [<cite idref="PUB00033963"/>].</p><p>This family describes the elongation factor SelB, a close homologue of EF-Tu. It may function by replacing EF-Tu. A C-terminal domain not found in EF-Tu is in all SelB sequences in the seed alignment except that from <taxon tax_id="2190">Methanocaldococcus jannaschii</taxon> (Methanococcus jannaschii). This family should not include an equivalent protein for eukaryotes.</p>