  The ratio between the anomalous magnetic moments of proton and neutron has
recently been suggested to be connected to the ratio of proton momentum
fractions carried by valence quarks. This relation has been obtained within a
parametrization of the Generalized Parton Distributions (GPD) \cite{gpv}, but
it is completely independent of such a parametrization.\\ It will be shown that
using different CQMs this relation holds within a few percent accuracy. This
agreement is based on what all the CQMs have in common: the effective degrees
of freedom of the three constituent quarks and the underlying SU(6) symmetry.\\
On the other hand, the experimental value of the ratio is not reproduced by
CQMs. This means that the SU(6)-breaking mechanism contained in the
phenomenological partonic distributions does not correspond to the SU(6)
breaking mechanism implemented in the CQMs we have analyzed \cite{noi03}.\\ We
will also show how this relation can be used in order to understand in which
way to implement an $SU(6)$-breaking mechanism and to test models.
