  Restrictions imposed on the (electromagnetic or weak) current operator by its
commutation relations with the representation operators of the Poincare group
are considered in detail. We argue that the present theory of deep inelastic
scattering based on perturbative QCD does not take into account the dependence
of the current operator on the nonperturbative part of the quark-gluon
interaction which cannot be neglected even in leading order in $1/Q$, where $Q$
is the magnitude of the momentum transfer.
