1. Given the set $M=\{1,3,5,7,9\}$. If the non-empty set $A$ satisfies: the elements of $A$ each increased by 4 form a subset of $M$, and the elements of $A$ each decreased by 4 also form a subset of $M$, then $A=$ $\qquad$
$-1 .\{5\}$.
$$
\begin{array}{l}
\text { Let } M_{1}=\{x \mid x=m-4, m \in M\} \\
=\{-3,-1,1,3,5\} , \\
M_{2}=\{x \mid x=m+4, m \in M\} \\
=\{5,7,9,11,13\} .
\end{array}
$$

From the problem, we know that $A \subseteq M_{1} \cap M_{2}=\{5\}$.
Since $A$ is a non-empty set, therefore, $A=\{5\}$.