We consider the gauge coupling running in a six-dimensional SO(10) orbifold
GUT model. The bulk gauge symmetry is broken down to the standard model gauge
group with an extra U(1)_X by orbifold boundary conditions and the extra U(1)_X
is further broken through the U(1)_{B-L} breaking with bulk hyper multiplets.
We obtain the corrections of Kaluza-Klein massive modes to the running of the
gauge couplings and discuss their implication to the successful gauge coupling
unification.Comment: 10 pages, 2 figures, version to appear in Physics Letters