  The totality of neutrino-oscillation phenomena appears to require the
existence of a light singlet neutrino. As pointed out recently, this can be
naturally accommodated with a specific extra U(1) factor contained in the
superstring-inspired E$_6$ model and its implied particle spectrum. We analyze
this model for other possible consequences. We discuss specifically the oblique
corrections from Z-Z' mixing, the phenomenology of the two-Higgs-doublet sector
and the associated neutralino sector, as well as possible scenarios of
gauge-coupling unification.
