react 使用ES6的写法，PropTypes为何不生效？

Question

import React from 'react';

import ReactDOM from 'react-dom';

let data = 123;

class MyTitle extends React.Component {

    //static propTypes = {

    //    title: React.PropTypes.string.isRequired

    //};//用这种方式则报错

    render() {

        return <h1> {this.props.title} </h1>;

    }

}

MyTitle.propTypes = {

    title: React.PropTypes.string.isRequired

};//验证不生效

ReactDOM.render(

    <MyTitle title={data}/>,

    document.getElementById('example')

);