Neutrino Masses and SO10 Unification

We present the embedding of the SM gauge group in SO10, a simple, compact unifying gauge group, with each of the three basic spin 1/2 families forming a unitary, irreducible 16-dimensional representation of spin10, which is complex, i.e. chiral. Subtle differences to the mixed representations of SU5, contained in the SO10 scheme, are pointed out. These have consequences for neutrino avors, which become paired in a light SU2_L− active doublet mode and a heavy SM singlet mode, one ν, N– pair per family.