  We show that the extension of the approximate custodial $SU(2)_{L+R}$ global
symmetry to all the Yukawa interactions of the standard model Lagrangian
implies the introduction of sterile right-handed neutrinos and the seesaw
mechanism in this sector. In this framework, the observed quark and lepton
masses may be interpreted as an effect of physics beyond the standard model.
The mechanism used for breaking this symmetry in the Yukawa sector could be
different from the one at work in the vector boson sector. We give three model
independent examples of these mechanisms.
