  Within a perturbative scalar QED model recently considered by Brodsky et al.,
we study how leading-twist Coulomb rescatterings affect the Drell-Yan cross
section at small x = x_{target}, and compare to the case of deep inelastic
scattering at small x_{B}. We show that in the range where the transverse
momentum transferred to the target is large compared to its minimal value $\sim
\morder{x}$, Coulomb rescatterings affect the DIS cross section but not the
Drell-Yan production rate. This illustrates that the leading-twist parton
distribution functions become non-universal when cross sections which are
differential in target-related particles are considered.
