Diphthamide is a unique posttranslationally modified histidine found only in translation elongation factor-2 (EEF2; MIM 130610). This modification is conserved from archaebacteria to humans and serves as the target for ADP-ribosylation and inactivation of EEF2 by diphtheria toxin (DT) and Pseudomonas exotoxin A. DPH4 is 1 of several enzymes involved in synthesis of diphthamide in EEF2 (Liu et al., 2004 [PubMed 15485916]).