  Using a quark model, we study the effect of charge symmetry breaking on the
valence quark distributions of the nucleon. The effect due to quark mass
differences and the Coulomb interaction of the electrically charged quarks is
calculated and, in contrast to recent claims, found to be small. In addition,
we investigate the effect of charge symmetry breaking in the confining
interaction, and in the perturbative evolution equations used to relate the
quark model distributions to experiment. We find that both these effects are
small, and that the strong charge symmetry breaking effect included in the
scalar confining interactions may be distinguishable from that generated by
quark mass differences.
