Flipped SU(5)
The Flipped SU(5) model is a Grand Unified Theory (GUT) theory first contemplated by Stephen Barr in 1982, and by Dimitri Nanopoulos and others in 1984. Antoniadis, Ellis, Hagelin and Nanopoulos developed the supersymmetric flipped SU(5), derived from the deeper-level superstring.
Current efforts to explain the theoretical underpinnings for observed neutrino masses are being developed in the context of supersymmetric flipped SU(5).
Flipped SU(5) is not a fully unified model, because the U(1)y factor of the SM gauge group is within the U(1) factor of the GUT group. The addition of states below Mx in this model, while solving certain threshold correction issues in string theory, makes the model merely descriptive, rather than predictive.
Read more about Flipped SU(5): The Model, Comparison With The Standard SU(5), Spacetime, Spatial Symmetry, Gauge Symmetry Group, Global Internal Symmetry, Vector Superfields, Chiral Superfields, Superpotential