The BFKL equation is based on the gluon yReggeization. In the leading and next-to-leading logarithmic approximations it is derived using the pole Regge form of QCD amplitudes with gluon quantum numbers in cross-channels and negative signature. This form is violated in the next-to-next-to-leading approximation. In two and three loops the observed violation can be explained by the presence of the three-Reggeon cut. Contributions of this cut to elastic scattering amplitudes up to four loops is discussed.