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Twitter is joining the S&P 500

Photo by Lars Niki/Corbis via Getty Images

Twitter shares climbed over 5% in Tuesday morning trading, as word came that it is being added to the S&P 500 index. Year-to-date, Twitter stock is up more than 60%.

What happened: The S&P 500 needed to replace Monsanto, which is being acquired later this week by Germany's Bayer.