Right Handed (RH) rotations are not observable in the standard model (SM). The freedom is used to give the phenomenologically correct mass matrices all kind of different forms and this is one of the reasons for the proliferation of models for the fermionic masses. The SM must be however extended and in most extensions, RH currents appear at higher energy scales. At those energies the RH rotations are not irrelevant any more, they can affect neutrino oscillations, proton decay, baryon assymetry, R-parity violating interactions etc. We study possible implications of large RH mixing in GUTs. Those are interesting not only because large mixing induce large effects. They are intimately related to large lepton mixing in GUTs, via a relation between LH mixing of the leptons and the RH ones of the d-quarks ,(``$d - \ell$ duality'') and can change considerably the branching ratios of proton decay. Observation of proton decay channels as well as neutrino oscillations will teach us about RH rotations and will reduce, therefore, considerably the freedom in the fermionic mass matrices. Some interesting examples are studied in detail. In particular a new $E_6$ model which realizes naturally a $d$ - $\ell$ duality by mixing with exotic $E_6$ fermions.