  Motivated by the desire to understand the nucleon mass structure in terms of
light-cone distributions, we introduce the twist-four parton distribution
function $F(x)$ whose first moment is the gluon condensate in the nucleon. We
present the equation of motion relations for $F(x)$ and discuss the possible
existence of the delta function (`zero mode') contribution at $x=0$. We also
perform one-loop calculations for quark and gluon targets.
