eIF5A (Elongation initiation factor 5A; also eIF5A1, Rev-binding factor and eIF4D) is a 16-19kD member of the eIF5A family of molecules. It is ubiquitously expressed, being found in all human cells examined. eIF5A acts as a stabilizer of RNA transcripts and acts as a positive modulator of p53mediated apoptosis. It also may be involved in the formation of the first peptide bond during select protein synthesis, and is known to be essential for cell cycle progression through G1/S. The human eIF5A precursor is 154aa in length. It contains a 71aa DOHH binding site (aa20-90) and a 69aa RNAbinding domain (aa84-152). eIF5A undergoes a posttranslational modification that converts the precursor peptide into a mature molecule. This modification occurs on 50aa, and involves the transfer and hydroxylation of an aminobutyl sequence from spermidine, creating a unique aa termed hypusine (hydroxyputrisine lysine). When hypusine is unacetylated, eIF5A is active; when acetylated, eIF5A is inactive Nonhypusinated eIF5A is found in both cytoplasm and nucleus, while hypusinated eIF5A is targeted to the cytoplasm. There is one isoform variant that shows a 32aa substitution for aa1-132. Over aa85-154, human and mouse are identical in aa sequence. Over the same aa sequence, human eIF5A (on Chr 1p13) also shares 98% and 69% aa identity with human eIF5A1-Like (on Chr 10q22) and human eiF5A2 (on Chr 3q26), respectively.