Hypusine [N epsilon-(4-amino-2-hydroxybutyl)-L-lysine] is a most remarkable amino acid, occurring in all eukaryotic cells, yet occupying only a single position in one protein, eukaryotic protein synthesis initiation factor 5A (eIF-5A). The unusual structure of hypusine, its derivation from the polyamine spermidine, and its increased formation in response to growth stimulation, as well as its limited occurrence in the highly conserved amino acid sequence of eIF-5A, have aroused keen interest in the biological significance of its existence and in its relationship to eIF-5A function.