We report the development of a new heterobase that is held together through reversible bonding. The so-formed cross-link adds strong stabilization to the DNA duplex. Despite this, the cross-link opens and closes through reversible imine bonding. Moreover, even enzymatic incorporation of the cross-link is possible. The new principle can be used to stabilize DNA duplexes and nanostructures that otherwise require high salt concentrations, which may hinder biological applications.