Answer: -3.
Solution: The condition is equivalent to the sum of the three numbers other than the maximum number among $1,2,3, x$ being 0. Clearly, $x<0$, so $1+2+x=0$, yielding $x=-3$.
1. Given the set of real numbers $\{1,2,3, x\}$, the maximum element of the set is equal to the sum of all elements in the set, then the value of $x$ is $\qquad$ .