  We study the model-building conditions under which an observable 0vBB decay
signal is predicted due to Kaluza-Klein singlet neutrinos in theories with
large extra dimensions. Our analysis is based on 5-dimensional singlet-neutrino
models compactified on an S^1/Z_2 orbifold, where the Standard-Model fields are
localized on a 3-brane. We show that sizeable 0vBB rates within the above
minimal 5-dimensional framework would require a non-vanishing shift of the
3-brane from the orbifold fixed points by an amount smaller than the typical
scale 100 inverse MeV characterizing the Fermi nuclear momentum. The resulting
5-dimensional models predict a sizeable effective Majorana-neutrino mass that
could be several orders of magnitude larger than the light neutrino masses.
