  A dominant QCD mechanism for the single transverse-spin asymmetry in hard
processes is induced by the twist-3 quark-gluon correlations inside nucleon,
combined with the soft-gluonic poles to produce the interfering phase for the
associated partonic hard scattering. It is shown that the corresponding
interfering amplitude can be calculated entirely in terms of the partonic Born
cross section which participates in the twist-2 cross section formula for the
spin-averaged process.
