It's been called the biggest biotech discovery of the century. And one company claims the largest market cap of any biotech focused on this major breakthrough.The discovery I'm referring to is CRISPR gene editing. There are only three biotechs that specialize in this approach to modifying DNA -- CRISPR Therapeutics (NASDAQ: CRSP), Editas Medicine (NASDAQ: EDIT), and Intellia Therapeutics (NASDAQ: NTLA).