  We discuss the isospin symmetry breaking (ISB) of the valence- and sea-quark
distributions between the proton and the neutron in the framework of the chiral
quark model. We assume that isospin symmetry breaking is the result of mass
differences between isospin multiplets and then analyze the effects of isospin
symmetry breaking on the Gottfried sum rule and the NuTeV anomaly. We show
that, although both flavor asymmetry in the nucleon sea and the ISB between the
proton and the neutron can lead to the violation of the Gottfried sum rule, the
main contribution is from the flavor asymmetry in the framework of the chiral
quark model. We also find that the correction to the NuTeV anomaly is in an
opposite direction, so the NuTeV anomaly cannot be removed by isospin symmetry
breaking in the chiral quark model. It is remarkable that our results of ISB
for both valence- and sea-quark distributions are consistent with the
Martin-Roberts-Stirling-Thorne parametrization of quark distributions.
