Hypusine dihydrochloride is a natural amino acid found only in the eukaryotic translation initiation factor 5A (eIF-5A). Hypusine is formed by the transfer of the butylamine portion from spermidine to the ϵ-amino group of a specific lysine residue of eIF-5A precursor and by the hydroxylation at carbon 2 of the incoming 4-aminobutyl moiety.