Perturbative QCD predictions for the small $x$ behaviour of unpolarized and polarized deep inelastic scattering structure functions

The perturbative QCD predictions for the small $x$ behaviour of the nucleon structure functions $F_{2,L}(x,Q^2)$ and $g_1(x,Q^2)$ are summarized. The importance of the double logarithmic terms for the small $x$ behaviour of the spin structure function $g_1(x,Q^2)$ is emphasised. These terms correspond to the contributions containing the leading powers of $\alpha_s ln(1/x)^2$ at each order of the perturbative expansion. In the non-singlet case they can be approximately accounted for by the ladder diagrams with quark (antiquark) exchange. We solve the corresponding integral equation with the running coupling effects taken into account and present estimate of the effective slope controlling the small $x$ behaviour of the non-singlet spin structure function $g_1(x,Q^2)$ of a nucleon.