We discuss results from our global QCD analyses including nuclear data
off deuterium from various measurements, as well as off ${}^3H$ and ${}^3He$ targets from the \mara{} 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 ${}^3He/{}^3H, {}^3He/{}^2H$, and ${}^3H/{}^2H$ cross section ratio are in good agreement with the predictions based on a single isoscalar off-shell function.
We provide the corresponding predictions on the ratio $F_2^n/F_2^p$, on the $d$ and $u$ quark distributions in the proton and in the ${}^3H$ and ${}^3He$ nuclei, as well as for future measurements of the EMC effect with parity-violating Deep Inelastic Scattering (DIS).