The incorporation of selenocysteine into a protein sequence is directed by an in-frame UGA codon (usually a stop codon) within the coding region of the mRNA. Selenoprotein mRNAs contain a conserved secondary structure in the 3' UTR that is required for the distinction of UGA stop from UGA selenocysteine. The selenocysteine insertion sequence (SECIS) is around 60 nt in length and adopts a hairpin structure which is sufficiently well-defined and conserved to act as a computational screen for selenoprotein genes.