  We discuss a grand unified theory (GUT) based on an $SO(32)$ GUT gauge group
broken to its subgroups including a special subgroup. In the $SO(32)$ GUT on
six-dimensional (6D) orbifold space $M^4\times T^2/\mathbb{Z}_2$, one
generation of the SM fermions can be embedded into a 6D bulk Weyl fermion in
the $SO(32)$ vector representation. We show that for a three generation model,
all the 6D and 4D gauge anomalies in the bulk and on the fixed points are
canceled out without exotic chiral fermions at low energies.
