  The grand unified theories are theoretically well motivated, but they
typically have less direct indications on the low energy physics and it is not
easy to test them. Here, we discuss a scenario of them which naturally solves
the so-called doublet-triplet splitting problem and, at the same time,
generally predicts characteristic collider phenomenology. Then, we may get a
hint on the breaking of the grand unified symmetry at the on-going and
next-generation collider experiments.
