  We perform the isospin decomposition of proton and neutron SLAC data for
moderately low $x$ in the region $0.01 \leq x \leq 0.1$. The isovector part is
well described by a power behaviour $x^{\alpha}$, where $\alpha$ does not
correspond to the intercept of single $a_1$ Regge trajectory, as expected.
However, the observed power leads to the validity of Bjorken sum rule and it is
consistent with the power extracted from all previous data using NLO evolution.
At the same time, the isoscalar part behaviour may be interpreted as a partial
cancellation between a positive non-singlet contribution and a strongly
negative singlet one. Further experimental consequences are mentioned.
