  Asymmetric nuclear matter at sub-saturation densities is shown to present
only one type of instabilities. The associated order parameter is dominated by
the isoscalar density and so the transition is of liquid-gas type. The
instability goes in the direction of a restoration of the isospin symmetry
leading to a fractionation phenomenon. These conclusions are model independent
since they can be related to the general form of the asymmetry energy. They are
illustrated using density functional approaches.
