and lepton masses and mixing angles become related to each other, and it begins to be possible to understand the spectrum. The simplest grand unified theory (GUT) is $SU(5)$ but this theory in its minimal version does not contain any right-handed neutrinos. Nevertheless three right-handed neutrinos may be added, and in this theory it is possible to have a large 23 element [^2] on the Dirac neutrino Yukawa matrix without introducing a large 23 element into any of the charged fermion Yukawa matrices. The problem of maintaining a 23 neutrino mass hierarchy in these models may be solved for example by assuming SRHND [@AlFe2]. Another possibility within the framework of $SU(5)$ is to maintain all the off-diagonal elements to be small, but require the 22 and 32 elements of the Dirac neutrino Yukawa matrix to be equal and the second right-handed neutrino to be dominant, in which case SRHND again leads to a large 23 neutrino mixing angle with hierarchical neutrino masses [@AlFeMa]. However the drawback of $SU(5)$ is that it does not predict any right-handed neutrinos, which must be added as an afterthought.

From the point of view of neutrino masses, the most natural GUTs are those like $SO(10)$ 