  The standard model of quarks and leptons is extended to include the gauge
symmetry $U(1)_\chi$ which comes from $SO(10) \to SU(5) \times U(1)_\chi$. The
radiative generation of Dirac neutrino masses through dark matter is discussed
in two examples. One allows for light Dirac fermion dark matter. The other
allows for self-interacting scalar dark matter with a light scalar mediator
which decays only to two neutrinos.
