In 2010, efforts to explain the theoretical underpinnings for observed neutrino masses were being developed in the context of supersymmetric flipped SU(5).[6]
Flipped SU(5) is not a fully unified model, because the U(1)Y factor of the Standard Model 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.[7]
^Barr, S.M. (1982). "A new symmetry breaking pattern for SO(10) and proton decay". Physics Letters B. 112 (3): 219–222. doi:10.1016/0370-2693(82)90966-2.