  We investigate the hyperon semileptonic decays and the quark spin content of
the proton $\Delta \Sigma$ taking into account flavor SU(3) symmetry breaking.
Symmetry breaking is implemented with the help of the chiral quark-soliton
model in an approach, in which the dynamical parameters are fixed by the
experimental data for six hyperon semileptonic decay constants. As a result we
predict the unmeasured decay constants, particularly for $\Xi^0 \to \Sigma^+$,
which will be soon measured and examine the effect of the SU(3) symmetry
breaking on the spin content $\Delta \Sigma $ of the proton. Unfortunately
large experimental errors of $\Xi^-$ decays propagate in our analysis making
$\Delta \Sigma$ and $\Delta s$ practically undetermined. We conclude that
statements concerning the values of these two quantities, which are based on
the exact SU(3) symmetry, are premature. We stress that the meaningful results
can be obtained only if the experimental errors for the $\Xi$ decays are
reduced.
