  A no-go theorem in flavor symmetries is reviewed. The theorem asserts that we
cannot bring any flavor symmetry into mass matrix model in which number of
Higgs scalars is, at most, one for each sector (e.g. H_u and H_d for up- and
down-quark sectors, respectively). Such the strong constraint comes from the
SU(2)_L symmetry. Possible three options to evade the theorem are discussed.
