[en] Models of the unification of the electroweak and the strong interaction predict that the neutrino is a Majorana particle and therefore essentially identical with its own antiparticle. In such grand unified models the neutrino has also a finite mass and a slight right-handed weak interaction, since the model is left-right symmetric. These models have also left-handed and right-handed vector bosons to mediate the weak interactions. If these models are correct the neutrinoless double beta-decay is feasible. Thus if one find the neutrinoless double beta-decay one knows that the standard model cannot be correct in which the neutrino is a Dirac particle and therefore different from its antiparticle. (author)