A See-Saw Model for Atmospheric and Solar Neutrino Oscillations
  We have constructed an explicit see-saw model containing two singlet neutrinos, one carrying a $(B-3L_e)$ gauge charge with an intermediate mass scale of $\sim O(10^{10})$ GeV along with a sterile one near the GUT (grand unification theory) scale of $\sim O(10^{16})$ GeV. With these mass scales and a reasonable range of Yukawa couplings, the model can naturally account for the near-maximal mixing of atmospheric neutrino oscillations and the small mixing matter-enhanced oscillation solution to the solar neutrino deficit. 
Ernest Ma, D.P. Roy and Utpal Sarkar