Gauge symmetry reduction from the extra space S-1/Z(2)

We study a mechanism of symmetry transition upon compactification of a 5-dimensional held theory on S-1/Z(2). The transition occurs unless all components in a multiplet of a symmetry group have a common Zz parity on S-1/Z(2). This mechanism is applied to a reduction of SU(5) gauge symmetry in grand unified theory, and phenomenological implications are discussed.