  The gluon distribution of a nucleon propagating through a nucleus can change
depending on how far the penetration depth is. It is a nonperturbative process
that we describe by an evolution equation. The kernel of the integral equation
is to be determined by a phenomenological study of J/psi suppression in pA
collision. The data of E866 on alpha(x_F) shows significant dependence on x_F
at large x_F. It presents a feature that has not been explained by any hadronic
absorption model. We show that gluon depletion is a simple mechanism that can
account for it. The result has far-reaching implications on the role of partons
in nuclear collisions.
