ReactJS Mixin Method Override

How can I create a method on a ReactJS Mixin such that I'm allowed to override it in the Component definition?

HTML

<script src="http://fb.me/JSXTransformer-0.10.0.js"></script>
<script src="http://fb.me/react-with-addons-0.10.0.js"></script>
<script src="http://fb.me/react-js-fiddle-integration.js"></script>
<div>hello<br />world<br /></div>
<div id='content'></div>

JavaScript 1.7

/** @jsx React.DOM */
 
var Main = React.createClass({
    getInitialState: function() {
        return {output: 'hello<br />world<br />'};
      },
    render: function() {
        return (
            <div>           
                {this.state.output}
            </div>
        );
        }
});

React.renderComponent(<Main/>, document.getElementById('content'));