  Comparison of structure functions measured in neutrino and charged lepton
deep inelastic scattering can be used to test basic properties of parton
distribution functions such as the validity of charge symmetry. Recent
experiments indicate a substantial discrepancy between $F_2^\nu$ and $F_2^\mu$
in the region of small Bjorken-x. We discuss nuclear corrections and strange
and anti strange quark effects and show that none of them can account for the
observed discrepancy. These results suggest surprisingly large CSV effects in
nucleon sea distributions.
