  We consider the mechanism of screening of the Dirac flavor structure in the
context of the double seesaw mechanism. As a consequence of screening, the
structure of the light neutrino mass matrix, m_\nu, is determined essentially
by the structure of the (Majorana) mass matrix, M_S, of new super-heavy (Planck
scale) neutral fermions S. We calculate effects of the renormalization group
running in order to investigate the stability of the screening mechanism with
respect to radiative corrections. We find that screening is stable in the
supersymmetric case, whereas in the standard model it is unstable for certain
structures of M_S. The screening mechanism allows us to reconcile the
(approximate) quark-lepton symmetry and the strong difference of the mixing
patterns in the quark and lepton sectors. It opens new possibilities to explain
a quasi-degenerate neutrino mass spectrum, special ``neutrino'' symmetries and
quark-lepton complementarity. Screening can emerge from certain flavor
symmetries or Grand Unification.
