  We discuss the influence of fixed target Drell-Yan data on the extraction of
parton distribution functions at next-to-next-to-leading order (NNLO) in QCD.
When used in a parton distribution fit, the Drell-Yan (DY) data constrain sea
quark distributions at large values of Bjorken x. We find that not all
available DY data are useful for improving the precision of parton distribution
functions (PDFs) obtained from a fit to the deep inelastic scattering (DIS)
data. In particular, some inconsistencies between DIS-based parton distribution
functions and DY data for large values of dilepton rapidity are found. However,
by selecting a sample of the DY data that is both representative and consistent
with the DIS data, we are able to perform a combined PDF fit that significantly
improves the precision of non-strange quark distributions at large values of x.
The NNLO QCD corrections to the DY process are crucial for improving the
precision. They reduce the uncertainty of the theoretical prediction, making it
comparable to the experimental uncertainty in DY cross-sections over a broad
range of x.
