A model of neutrino mixing based on an $A_4$ flavor symmetry is suggested. Other possible models are also listed or briefly reviewed. Besides the fields in the standard model, these models also contain new fields that transform under different representations of the group $A_4$.
To explain the current experimental data, the present model is constructed to deviate from a tri-bimaximal model just slightly; hence, all analysis can be done on the base of the perturbation method. Numerical results have shown the model is quite consistent with the current experimental data.
A relation to physics at Belle-II is also discussed