  We calculate twist-3 parton ditribution functions (PDFs) using cut and uncut
diagrams. Uncut diagrams lead to a Dirac delta function term. No such term
appears when cut diagrams are used. We show that a $\delta(x)$ is necessary to
satisfy the Lorentz invariance relations of twist-3 PDFs, except for the
Burkhardt-Cottingham sum rule in QCD.
