  We present a new solution to the doublet-triplet splitting problem which also
works in a supersymmetric SU(5) grand unified theory and is testable at
TeV-scale collider experiments. In our model, the SU(5) symmetry is broken
through the Hosotani mechanism. Thanks to the phase nature of the
Hosotani-breaking, the "missing VEV" can be realized even in an SU(5) model. A
general and distinctive prediction of this solution is the existence of light
adjoint chiral supermultiplets with masses of the supersymmetry breaking scale.
Though these disturb the gauge coupling unification, it can be recovered
keeping the unified gauge coupling constant perturbative by introducing
additional vector-like particles, which may be also observed in the upcoming
collider experiments.
