  The treatment of the running QCD coupling in evolution equations is
discussed. It is shown that the use of the virtuality of ladder (vertical)
partons as the scale for QCD coupling in every rung of ladder graphs is an
approximation that holds for DIS at large x only. On the contrary, in the small
x region the coupling depends on the virtuality of s -channel (horizontal)
gluons. This observation leads to different results for the Regge-like
processes and DIS structure functions at small x.
