  Recent data from astrophysical and terrestrial experiments indicates large
mixing angles in the neutral lepton sector and restricts the allowed regions of
neutrino masses. In particular, the large mixing angles in the lepton sector
are disparate from the small mixing in the quark sector. This disparity is
unnatural from the point of view of grand unified theories, that are well
motivated by the Standard Model multiplet structure and logarithmic running of
its parameters. we examine the issue of this disparity from the perspective of
string derived SO(10) GUT models, in which the SO(10) symmetry is broken
directly at the string theory level. A characteristic feature of such models is
the appearance of numerous SO(10) singlet fields. We propose that the mismatch
between the quark and lepton mixing parameters arises due to this extended
singlet spectrum and its mixing with the right-handed neutrinos. We discuss a
string inspired effective parameterization of the extended neutrino mass matrix
and demonstrates that the coupling with the SO(10) singlet spectrum can readily
account for the neutrino flavor parameters. The mechanism implies that some
SO(10) singlet fields should exist at intermediate mass scales. We study the
possibilty of deriving the neutrino mass spectrum from string theory in a
specific string derived vacuum solution, and comment on the properties that
such a solution should possess.
