Apart from with all that, there is also the dynamical aspect of the local S U ( 2 ) F symmetry, related to its horizontal gauge bosons. The point is, however, that these bosons (as well as various Higgs bosons involved) have to be several orders of magnitude more massive than the Standard Model W and Z bosons in order to avoid forbidden quark-flavor- and lepton-flavor-changing transitions. Generally, this requires the introduction of additional Higgs bosons to give the large masses to the horizontal gauge bosons so as to not disturb the masses of the fermions involved.