  Since the main experimentally testable prediction of grand unified theories
is the instability of the proton, precise determination of the proton lifetime
for each particular model is desirable. Unfortunately, the corresponding
computation usually involves theoretical uncertainties coming e.g. from
ignorance of the mass spectrum or from the Planck-suppressed higher-dimensional
operators, which may result in errors in the proton lifetime estimates
stretching up to several orders of magnitude. On the other hand, we present a
model based on SO(10) gauge group which is subsequently broken by a scalar
adjoint representation, where the leading Planck-suppressed operator is absent,
hence the two-loop precision may be achieved.
