Cover photograph (Copyright © 2001, American Society for Microbiology. All rights reserved.): Selenocysteine, the 21st amino acid in protein, is incorporated cotranslationally at UGA codons. Selenocysteyl-tRNA is recognized by a unique elongation factor, EFsec (small blue ovals), and recruited to selenoprotein mRNAs via interactions between EFsec, SECIS binding protein 2 (SBP2, red ovals), and the SECIS element in the 3' untranslated region (black stem-loop). Isopentenylation of adenosine at position 37 (i⁶A, top), immediately 3' of the anticodon in selenocysteine tRNA, is required for the tRNA to function. Substitution of G for A prevents isopentenylation (i⁶A, bottom), inhibiting selenoprotein synthesis. Ribosomes are depicted as large blue ovals, selenocysteine (sec) and the nascent polypeptide chain are shown in yellow, tRNAs (3'-to-5' direction) are depicted in orange, and mRNA (5'-to-3' direction) is depicted in black. (See related article in June 2001, issue 11: vol. 21, p. 3852.)