  Near maximal neutrino mixing needed to understand atmospheric neutrino data
can be interpreted to be a consequence of an interchange symmetry between the
muon and tau neutrinos in the neutrino mass matrix in the flavor basis. This
idea can be tested by a measurement of the neutrino mixing parameter
$\theta_{13}$ and looking for its correlation with $\theta_{23}-\pi/4$. We
present a supersymmetric SU(5) grand unified model for quarks and leptons which
obeys this exchange symmetry and is a realistic model that can fit all
observations. GUT embedding shifts $\theta_{13}$ from its zero $\mu-\tau$
symmetric value to a nonzero value keeping it under an upper limit.
