  In light of the observation of a relatively large $\theta^{}_{13}$, one has
to consider breaking the $\mu$-$\tau$ symmetry properly which would otherwise
result in a vanishing $\theta^{}_{13}$ (as well as $\theta^{}_{23} = \pi/4$).
Therefore, we investigate various symmetry-breaking patterns and accordingly
identify those that are phenomenologically viable. Furthermore, the
symmetry-breaking effects arising from some specific physics (e.g., the
renormalization group equation running effect) are discussed as well.
