  We extract the ratio of the down (d) and up (u) parton distribution functions
(PDF's) from the ratio of deuteron and proton structure function F_2(d)/F_2(p)
measured by NMC. We use corrections for nuclear binding effects in the
deuteron, which are extracted from the nuclear dependence of SLAC F_2 data.
Significant corrections to the d quark distribution in standard PDF's are
required, especially at high x. The corrected d/u ratio is in agreement with
the QCD prediction of 0.2 at x=1. The predictions for the W asymmetry in hadron
colliders using PDF's with the corrected d/u ratio are in much better agreement
with recent CDF data at large rapidity. Using the updated d/u ratio and the
most recent world average for alpha_s, we perform a NLO global fit to all DIS
data for F_2 and R, and estimate the size of the higher twist contributions
using both a renormalon model and an empirical model. We find that with the
updated value of alpha_s, the magnitude of the higher twist terms is half the
value of previous analysis. With the inclusion of target mass and higher twist
corrections, the standard NLO PDF's with the updated d/u ratio describe the
SLAC F_2 data up to x=1.0. When the analysis is repeated in NNLO, we find that
the additional NNLO contributions to R account for most of the higher twist
effects extracted in the NLO fit. The analysis in NNLO indicates that the
higher twist effects in R, F_2 and xF_3 (e.g. GLS sum rule) are very small.
