III. THE LEPTONIC SPECTRUM 9 mode between ζ$_{1}$ and ζ$_{2}$ , and y α s splits the RH charge − 1 singlet between ζ$_{3}$ and ζ$_{α}$ . This splitting is crucial in models of GHU, since the Higgs being part of a gauge multiplet can only mix fermion fields within the same SO (5) multiplet, coupling to them with the same (gauge) strength. The non-trivial flavor structure is then only due to the brane terms above. Thus, the only source of flavor violation in the rotated basis comes from ˆ θ$_{M}$ in Eq. (16), whose particular form will eventually lead to TBM mixing in the leptonic sector. This flavor universality is a welcome consequence of the A$_{4}$ symmetry, for it will also prevent flavor violating operators mediated by heavy KK gauge bosons to exceed current experimental bounds. This observation, which was made in simpler models with warped extra dimen- sions [29], is maintained at this order in the more realistic models with GHU under study here. Incidentally, the extra fields required to complete the SO (5) representations imply that simpler Z$_{2}$ or Z$_{3}$ symmetries are not suitable to banish operators violating this mixing pattern. In order to find the lepton masses and mixings we have to solve the equations of motion derived from the bulk action with the boundary conditions in Eqs. (17) and (18). This can be actually carried out exactly in the case of GHU models because the Higgs, which is part of a higher-dimensional gauge field, can be eliminated from the bulk by a rotation in gauge space, thus reducing the Higgs effect to the modification of the boundary conditions. This is essential, for otherwise the Higgs would mix different multiplets in the bulk, and the corresponding equations of motion would be forbiddingly difficult to solve. Still, the large number of fields involved makes the solution of the full system technically challenging. An alternative approach is to perform a Kaluza-Klein (KK) expansion without including the Higgs, and then to take into account its effects by diagonalizing the corresponding mass matrix. In this case one must include the Majorana masses not in the KK expansion but as a contribution to the mass matrix. Otherwise we would have to incorporate the effect of all physical modes up to the Majorana mass scale (which is ∼ R − $^{1}$) in order to obtain an accurate enough approximation [21]. Furthermore, the leading contribution to the light neutrino masses and mixing angles can be obtained by simply considering the zero modes in the KK expansion (thus including the heavy Majorana RH neutrinos), for heavier KK modes