  We show that 1-loop transitions between two quasi-degenerate fermions can
induce a potentially large renormalization of their mixing angle, and a large
renormalized Cabibbo (or PMNS) angle when the second fermion pair in the same
two generations is far from degeneracy. At the resonance, the "Cabibbo angle"
gets maximal and simply connected to masses without invoking any new physics
beyond the standard model. This solution appears as the only one
"perturbatively stable" (mixing angles are then renormalized with respect to
their classical values by small amounts).
