Abstract
The three-generation fermions should not be included in grand unified theories (GUT's) if only one extra $Z$ boson exists. The minimal GUT's with three generations should include two extra $Z$ bosons which belong to different broken scales. Georgi's argument on heavy Dirac fermions is realized. These Dirac fermions should not be bizarre. Frampton's SU(9) model should be essentially the same as his SU(7) model. The difficulty of proton decay may be resolved.
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