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>

JavaScript 1.7

/** @jsx React.DOM */

var MyMixin = {
  render: function() {
    return this.renderItem();
  }
  
};

var Hello = React.createClass({
  mixins: [MyMixin],
  renderItem: function() {
    return <div > Hey {
      this.props.name
    } < /div>;
  }
});

React.renderComponent( < Hello name = "World" / > , document.body);

// Notice in the console an error is logged: 

/*
Uncaught Error: Invariant Violation: ReactCompositeComponentInterface: You are attempting to define `renderItem` on your component more than once. This conflict may be due to a mixin.
*/