  The Gottfried sum rule violation reported by the New Muon Collaboration(NMC)
was interpreted as indication for a flavour asymmetry of the sea quark in the
nucleons. We investigate the alternative possibility that isospin symmetry
between the proton and the neutron is breaking. We examine systematically the
consequences of this possibility for several processes, namely, neutrino deep
inelastic scattering, charged pion Drell-Yan process, proton Drell-Yan process,
and semi-inclusive deep inelastic scattering, and conclude that a decision
between the two alternative explanations is possible.
