  We consider the conditional regularity of mild solution $v$ to the
incompressible Navier-Stokes equations in three dimensions. Let $e \in
\mathbb{S}^2$ and $0 < T^\ast < \infty$. J. Chemin and P. Zhang \cite{CP}
proved the regularity of $v$ on $(0,T^\ast]$ if there exists $p \in (4, 6)$
such that $$\int_0^{T^\ast}\|v\cdot e\|^p_{\dot{H}^{\frac{1}{2}+\frac{2}{p}}}dt
< \infty.$$ J. Chemin, P. Zhang and Z. F. Zhang \cite{CPZ} extended the range
of $p$ to $(4, \infty)$. In this article we settle the case $p \in [2, 4]$. Our
proof also works for the case $p \in (4,\infty)$.
