  The constraints of gauge unification on intermediate mass scales in
non-supersymmetric SO(10) scenarios are systematically discussed. With respect
to the existing reference studies we include the U(1) gauge mixing
renormalization at the one- and two-loop level, and reassess the two-loop
beta-coefficients. We evaluate the effects of additional Higgs multiplets
required at intermediate stages by a realistic mass spectrum, and update the
discussion to the present day data. On the basis of the obtained results,
SO(10) breaking patterns with up to two intermediate mass scales are discussed
for potential relevance and model predictivity.
