  Deep inelastic scattering of $e^{\pm}$ off protons is sensitive to
contributions from "dark photon" exchange. Using HERA data fit to HERA's parton
distribution functions, we obtain the model-independent bound $\epsilon
\lesssim 0.02$ on the kinetic mixing between hypercharge and the dark photon
for dark photon masses $\lesssim 10$ GeV. This slightly improves on the bound
obtained from electroweak precision observables. For higher masses the limit
weakens monotonically; $\epsilon \lesssim 1$ for a dark photon mass of $5$ TeV.
Utilizing PDF sum rules, we demonstrate that the effects of the dark photon
cannot be (trivially) absorbed into re-fit PDFs, and in fact lead to non-DGLAP
(Bjorken $x_{\rm B}$-independent) scaling violations that could provide a
smoking gun in data. The proposed $e^\pm p$ collider operating at $\sqrt{s} =
1.3$ TeV, LHeC, is anticipated to accumulate $10^3$ times the luminosity of
HERA, providing substantial improvements in probing the effects of a dark
photon: sensitivity to $\epsilon$ well below that probed by electroweak
precision data is possible throughout virtually the entire dark photon mass
range, as well as being able to probe to much higher dark photon masses, up to
$100$ TeV.
