The eukaryotic initiation factor 5A (eIF5A) contains a polyamine-derived amino acid, hypusine [N(ε)-(4-amino-2-hydroxybutyl)lysine]. Hypusine is formed post-translationally by the addition of the 4-aminobutyl moiety from the polyamine spermidine to a specific lysine residue, catalyzed by deoxyhypusine synthase (DHPS), and subsequent hydroxylation by deoxyhypusine hydroxylase (DOHH). The eIF5A precursor protein and both of its modifying enzymes are highly conserved, suggesting a vital cellular function for eIF5A and its hypusine modification. To address the functions of eIF5A and the first modification enzyme, DHPS, in mammalian development, we knocked out the Eif5a or the Dhps gene in mice. Eif5a heterozygous knockout mice and Dhps heterozygous knockout mice were viable and fertile. However, homozygous Eif5a1 (gt/gt) embryos and Dhps (gt/gt) embryos died early in embryonic development, between E3.5 and E7.5. Upon transfer to in vitro culture, homozygous Eif5a (gt/gt) or Dhps (gt/gt) blastocysts at E3.5 showed growth defects when compared to heterozygous or wild type blastocysts. Thus, the knockout of either the eIF5A-1 gene (Eif5a) or of the deoxyhypusine synthase gene (Dhps) caused early embryonic lethality in mice, indicating the essential nature of both eIF5A-1 and deoxyhypusine synthase in mammalian development.