  In 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 GUT-threshold,gravitational
smearing,and string threshold effects. 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 $M_I \simeq 5\times
10^{12}$,needed for neutrino masses,with the GUT scale $M_U \simeq M_{str}$ can
be realized provided certain particle states in the predicted spectum are
light.
