I am struggling with a problem like this: In dimension $n\geq 3$,
For the following uniformly elliptic equation, do we have interior gradient estimates?
$$a^{ij}(x)u_{ij}(x)+u_{nn}=0.$$, where $i,j=1,\cdots, n-1.$ 
$\lambda id<(a^{ij})<\Lambda id$, namely the equation is uniformly elliptic. Is
there any interior gradient estimates? More specifically, I need the estimate
$|u_n|\leq C$ in the interior, where $C$ depends only on $|u|_{L^\infty}$ and
the elliptic constant.