SECIS elements form stem-loop structures in the 3' untranslated regions (UTR) of 
eukaryotic mRNAs that encode selenoproteins. These elements direct incorporation 
of selenocysteine at UGA codons, provided the SECIS element lies a sufficient 
distance from the UGA. The cDNAs encoding skeletal muscle selenoprotein W from 
human, rhesus monkey, sheep, rat, and mouse contained highly similar SECIS 
elements that retained important features common to all known SECIS elements. 
Comparative analysis of these SECIS elements showed that in some regions both 
predicted secondary structure and nucleotide sequences were conserved, in other 
areas secondary structure was maintained using different primary sequence, and 
in still other portions, base pairing was not conserved. The rodent and sheep 
selenoprotein W mRNAs used UGA as a stop codon and as a selenocysteine codon. 
Thus, UGA specified both selenocysteine incorporation and termination in a 
single mRNA. The selenoprotein W SECIS elements contained an additional highly 
conserved base-paired stem that may prevent inappropriate selenocysteine 
incorporation at the UGA stop codons.