  A surprisingly large charge symmetry violation of the sea quarks in the
nucleon has been proposed in a recent article by Boros et al. as an explanation
of the discrepancy between neutrino (CCFR) and muon (NMC) nucleon structure
function data at low x. We show that these models are ruled out by the
published CDF W charge asymmetry measurements, which strongly constrain the
ratio of d and u quark momentum distributions in the proton over the x range of
0.006 to 0.34. This constraint also limits the systematic error from possible
charge symmetry violation in the determination of sin^2(theta) from nu-N
scattering experiments.
