  We present an improved determination of the up- and down-quark distributions
in the proton using recent data on charged lepton asymmetries from $W^\pm$
gauge-boson production at the LHC and Tevatron. The analysis is performed in
the framework of a global fit of parton distribution functions. The fit results
are consistent with a non-zero iso-spin asymmetry of the sea, $x(\bar d - \bar
u)$, at small values of Bjorken $x\sim 10^{-4}$ indicating a delayed onset of
the Regge asymptotics of a vanishing $(\bar d - \bar u)$-asymmetry at
small-$x$. We compare with up- and down-quark distributions available in the
literature and provide accurate predictions for the production of single
top-quarks at the LHC, a process which can serve as a standard candle for the
light quark flavor content of the proton.
