2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/43215Extra dimension provides an attractive way to break symmetry by boundary conditions which can be useful to construct a natural grand unified theory avoiding the doublet-triplet splitting problem and the proton decay problem. It can provide also an elegant mechanism to generate hierarchical 4-dimensional Yukawa couplings, involving the quasi-localization of matter fields in extra dimension. We discuss the Kaluza-Klein threshold corrections to low energy gauge couplings in generic 5-dimensional orbifold grand unified theories with quasi-localized matter fields, and apply the results to some class of SU(5) models on $S^1/Z_2\times Z_2$.5 pages, Revtex4, To appear in the Proceedings of SUSY03, held at the University of Arizona, Tucson, AZ, USA, June 5-10, 2003High Energy Physics - Phenomenologytext