Vanishing corrections on an intermediate scale and implications for unification of forces

In a two-step breaking of a class of grand unified theories including SO(10), we prove a theorem showing that the scale (M-I), where the Pati-Salam gauge symmetry with parity breaks down to the standard gauge group, has vanishing corrections due to all sources emerging from higher scales (mu>M-I) such as the one-loop and all higher-loop effects, the grand unified theory threshold, gravitational smearing, and string threshold effects. The implications of such a scale for the unification of gauge couplings with small Majorana neutrino masses are discussed. In string inspired SO(10), we show that M-I similar or equal to 5x10(12) GeV, needed for neutrino masses, with the GUT scale M-U similar or equal to M-str, can be realized provided certain particle states in the predicted spectrum are light.