Deoxyhypusine is a highly rare posttranslational modification that occurs in eukaryotes where the it is vital for the formation of the initiation factor eIF5A. The enzyme carrying the reaction out is deoxyhypusine synthase which is able to transfer a butylamine moiety from spermidine to a lysine in eIF5A, using NAD as a cofactor. Since active eIF5A is required for tumour cell proliferation, design of inhibitors for this enzyme could prove useful in the treatment of certain cancers. The high specificity of the enzyme is also of interest, because the only incidence of this modification in eukaryotic proteins appears to be in this particular case.