The curvature of $F_2^p(x,Q^2)$ as a probe of the range of validity of   perturbative QCD evolutions in the small-$x$ region
  Perturbative NLO and NNLO QCD evolutions of parton distributions are studied, in particular in the (very) small-$x$ region, where they are in very good agreement with all recent precision measurements of $F_2^p(x,Q^2)$. These predictions turn out to be also rather insensitive to the specific choice of the factorization scheme ($\bar{\rm MS}$ or DIS). A characteristic feature of perturbative QCD evolutions is a {\em{positive}} curvature of $F_2^p$ which increases as $x$ decreases. This perturbatively stable prediction provides a sensitive test of the range of validity of perturbative QCD. 
M. Gl\"uck, C. Pisano, E. Reya