  Using a quark model, we calculate symmetry breaking effects in the valence
quark distributions of the nucleon. In particular, we examine the breaking of
the quark model SU(4) symmetry by color magnetic effects, and find that color
magnetism provides an explanation for deviation of the ratio $d_V(x)/u_V(x)$
from $1/2$. Additionally, we calculate the effect of charge symmetry breaking
in the valence quark distributions of the proton and neutron and find, in
contrast to other authors, that the effect is too small to be seen
experimentally.
