  Studying the possible breaking of various parton model symmetries by the
parton distribution functions of the nucleon can provide important information
for the non-perturbative structure of hadrons and the strong interaction. We
review theoretical calculations for the breaking of flavor symmetry,
quark-antiquark symmetry and charge symmetry in the unpolarized and polarized
nucleons using the meson cloud model. We report an estimation for the total
distribution of strange and antistrange quarks in the nucleon by combining
theoretical calculations of SU(3) flavor symmetry breaking with light antiquark
distributions obtained from global analysis of available experimental data.
