We demonstrate that perturbative QCD leads to positive 3D parton--parton correlations inside nucleon explaining a factor two enhancement of the cross section of multi-parton interactions observed at Tevatron at $x_ige 0.01$ as compared to the predictions of the independent parton approximation. We also find that though perturbative correlations decrease with $x$ decreasing, the nonperturbative mechanism kicks in and should generate correlation which, at $x$ below $10^{-3}$, is comparable in magnitude with the perturbative one for $xsim 0.01$.
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