Parton Distributions,d/u, and Higher Twist Effects at Highx

A reanalysis of the NMC and SLAC data leads to a great improvement in our knowledge of the valence $d$ and $u$ parton distribution functions (PDF's) at high $x$. Standard parton distributions with our modifications are in good agreement with QCD predictions for $d/u$ at $x=1\mathrm{}$, and with the CDHSW $\nu p$ and $\overline{\nu }p$ data, the DESY HERA charged current cross section data, the collider high- $P_t$ jet data, and the CDF $W$ asymmetry data. With the inclusion of target mass and higher twist corrections, the modified PDF's also describe all deep-inelastic scattering data up to $x=0.98\mathrm{}$ and down to $Q^2=1\mathrm{}\mathrm{GeV}{}^2$.