Broken $S_3$ Symmetry in Flavor Physics

The $S_3$ symmetry is shown to be a very good approximate symmetry when it is broken in a specific way. This is true both in quark sector and in lepton sector. The way to break it is implied by the K-M mechanism applied not to the mixing matrix but to the mass matrices. In quark sector, we have an almost perfect fitting to the experimental data, and in lepton sector, we have a precision for the $\theta_{13}$.