  We critically examine uncertainties in parton distribution functions (PDFs)
at large x arising from nuclear effects in deuterium F2 structure function
data. Within a global PDF analysis, we assess the impact on the PDFs from
uncertainties in the deuteron wave function at short distances and nucleon
off-shell effects, the use of relativistic kinematics, as well as the use of
less a restrictive parametrization of the d/u ratio. We find that in particular
the d-quark and gluon PDFs vary significantly with the choice of nuclear model.
We highlight the impact of these uncertainties on the determination of the
neutron structure function, and on W boson production and parton luminosity at
the Tevatron and the LHC. Finally, we discuss prospects for new measurements
sensitive to the d-quark and gluon distributions but insensitive to nuclear
corrections.
