  We discuss results from our global QCD analyses including nuclear data off
deuterium from various measurements, as well as off $^3$H and $^3$He targets
from the MARATHON experiment. We simultaneously determine the parton
distribution functions of the proton, the higher-twist terms, and the nucleon
off-shell correction functions responsible for the modifications of the
partonic structure in bound protons and neutrons. In particular, we study the
neutron-proton asymmetry of the off-shell correction and its interplay with the
treatment of the higher-twist terms. We observe that the data on the
$^3$He/$^3$H cross section ratio are consistent with a single isoscalar
off-shell function. We also provide our predictions on the ratio $F_2^n/F_2^p$
and on the $d$ and $u$ quark distributions in the proton and in the $^3$H and
$^3$He nuclei.
