  It is now widely recognized that a key to unravel the nonperturbative
chiral-dynamics of QCD hidden in the deep-inelastic-scattering observables is
the flavor structure of sea-quark distributions in the nucleon. We analyze the
flavor structure of the nucleon sea in both of the unpolarized and
longitudinally polarized parton distribution functions (PDFs) within a single
theoretical framework of the flavor SU(3) chiral quark soliton model (CQSM),
which contains only one adjustable parameter $\Delta m_s$, the effective mass
difference between the strange and nonstrange quarks. A particular attention is
paid to a nontrivial correlation between the flavor asymmetry of the
unpolarized and longitudinally polarized sea-quark distributions and also to a
possible particle-antiparticle asymmetry of the strange quark distributions in
the nucleon. We also investigate the charge-symmetry-violation (CSV) effects in
the parton distribution functions exactly within the same theretical framework,
which is expected to provide us with valuable information on the relative
importance of the asymmetry of the strange and antistrange distributions and
the CSV effects in the valence-quark distributions inside the nucleon in the
resolution scenario of the so-called NuTeV anomaly in the extraction of the
Weinberg angle.
