  We present a model for inclusive charged lepton-nucleon and
(anti)neutrino-nucleon cross sections at momentum transfer squared, $Q^2$,
$\sim1 {\rm GeV}^2$. We quantify the impact of existing low-Q charged-lepton
deep-inelastic scattering (DIS) data on effects due to high-twist operators and
on the extraction of parton distribution functions (PDFs). No evidence is found
for twist-6 contributions to structure functions (SF), and for a twist-4 term
in the logitudinal SF at $x\gtrsim0.1$. We find that DIS data are consistent
with the NNLO QCD approximation with the target mass and phenomenological high
twist corrections. For $Q^2<1 {\rm GeV}^2$, we extend extrapolation of the
operator product expansion, preserving the low-$Q$ current-conservation
theorems. The procedure yields a good description of data down to $Q^2\sim 0.5
{\rm GeV}^2$. An updated set of PDFs with reduced uncertainty and applicable
down to small momentum transfers in the lepton-nucleon scattering is obtained.
