Stability of Neutrino Mass Degeneracy

Two neutrinos of Majorana masses $m_{1,2}$ with mixing angle $\theta$ are unstable against radiative corrections in the limit $m_1 = m_2$, but are stable for $m_1 = -m_2$ (i.e. opposite CP eigenstates) with $\theta = 45^\circ$ which corresponds to an additional symmetry.