Grand-Unification Scale Generation through the Anomalous U(1) Breaking

We discuss the anomalous U(1) gauge symmetry as a mechanism of generating the grand-unification scale. We conclude that unification to a simple group cannot be realized unless some parameters are ``tuned'', and that models with product gauge groups are preferred. We consider the ``R-invariant natural unification'' model with gauge groups SU(5)_{GUT} \times U(3)_H. In this model the doublet-triplet splitting problem is solved and the unwanted GUT relation m_s = m_\mu is avoided maintaining m_b = m_\tau. Moreover, R-invariance suppresses the dangerous proton decays induced by dimension four and five operators.