  Based on the idea that electromagnetism is responsible for the mass
differences within isotopic multiplets, and possibly also for the whole mass of
the electron, a supersymmetric gauge theoretical model based on the group
$SU(2)_{L} \times SU(2)_{R} \times U(1)_{Y}$ is constructed. Under some
reasonable assumptions to the SUSY particle spectrum, a correct sign for the
mass difference within an isotopic multiplet is obtained. This might provide a
possible scenario to understand the old puzzle of the proton-neutron mass
difference.
