  Precise measurements of electron and positron scattering, including parity
violation, offer great promise in the search for physics beyond the Standard
Model. In this context it is crucial to understand the corrections which might
arise from charge symmetry violation, as well as the less well known strange
and charm quark distributions. Our analysis, using state of the art parton
distributions, suggests that these contributions lead to corrections in the
extraction of the weak couplings $g^{eq}_{AV}$ and $g^{eq}_{VA}$ of the order
$(1-2)\%$, while they are as large as $4\%$ for $g^{eq}_{AA}$, at a typical
scale of $Q^2 = 10\ {\rm GeV}^2$. These results underline the importance of
carrying out high precision measurements, which will not only provide
information on physics beyond the Standard Model but also reduce the current
uncertainties on our knowledge of the strange and charm quark distributions in
the proton.
