  The small mass difference $m_n-m_p=1.3 MeV$ between the proton and neutron
leads to an excess of $n=\pi^- p$ over $p=\pi^+ n$ fluctuations which can be
calculated by using a light-cone meson-baryon fluctuation model of intrinsic
quark-antiquark pairs of the nucleon sea. The Gottfried sum rule violation may
partially be explained by isospin symmetry breaking between the proton and
neutron and the same effect introduces correction terms to the anomalous
magnetic moments and the anomalous weak magnetic moments of the proton and
neutron. We also evaluated the strange magnetic moment of the nucleon from the
lowest strangeness $K \Lambda$ fluctuation and found a non-trivial influence
due to isospin symmetry breaking in the experimental measurements of the
strange magnetic moment of the nucleon.
