What do mice symbolize in Of Mice and Men?

Mice have a number of symbolic interpretations in Of Mice and Men. First of all, let us look at the title of the novel. The title is an allusion to Robert Burns' poem "To a Mouse," in which a mouse dreams of shelter and security but its "best laid schemes" do not quite come to fruition.
The mouse, therefore, acts as a metaphor for George and Lennie's dream to have a ranch of their own and to live off the land. Just like the mouse in the poem, no matter how hard they try, their dream is unachievable because of forces beyond their control.
Secondly, mice are also important to Lennie. In particular, we see that mice provide comfort to Lennie, as shown by the dead mouse he carries around in chapter 1. They are a source of comfort and pleasure in an uncertain world.
Moreover, the fact that Lennie kills the mice he strokes is symbolic of his uncontrollable strength. Even though Lennie has no wish to hurt another creature, he cannot stop himself from crushing mice under the weight of his hand, a sign that he will always encounter trouble because he lacks the mental ability to bring his strength under control.