N1-Guanyl-1,7-Diaminoheptane, also known as GC7, is the most efficient inhibitor of deoxyhypusine synthase (DHS) known. As a competitive inhibitor it binds to the active site of DHS and prevents the first step in the post-translational conversion of a single lysine to hypusine in translation initiation factor 5A (eIF5A). The modification is essential for sustained cell growth. GC7 acts on purified DHS and penetrates cells well to inhibit DHS inside the cell. GC7 has been used as cell cycle synchronizing reagent.