  World data on the lepton-charge asymmetry in the elastic and inelastic lepton
scattering off the proton and nuclei are compiled and discussed. After
reviewing the published results, we compare the elastic data to a model
calculation of the two-photon exchange mechanism. We show that the existing
data do not provide any evidence for the two-photon contribution. At
significance level 0.05 the data allow to exclude the two-photon exchange as an
explanation for the difference between Rosenbluth and polarization measurements
of proton electromagnetic form factors.
