  Let $n$ and $s$ be fixed integers such that $n\geq 2$ and $1\leq s\leq
\frac{n}{2}$. Let $M_n(\mathbb{K})$ be the ring of all $n\times n$ matrices
over a field $\mathbb{K}$. If a map $\delta:M_n(\mathbb{K})\rightarrow
M_n(\mathbb{K})$ satisfies that $\delta(xy)=\delta(x)y+x\delta(y)$ for any two
rank-$s$ matrices $x,y\in M_n(\mathbb{K})$, then there exists a derivation $D$
of $M_n(\mathbb{K})$ such that $\delta(x)=D(x)$ holds for each rank-$k$ matrix
$x\in M_n(\mathbb{K})$ with $0\leq k\leq s$.
