Image source: Flickr user stockmonkeys.com.Patent expiration risk means that today's biotech leaders may not be the leaders of tomorrow, but despite that risk, Gilead Sciences (NASDAQ: GILD), Celgene Corp. (NASDAQ: CELG), and Regeneron Pharmaceuticals, Inc. (NASDAQ: REGN) appear to have the staying power necessary to make them core holdings in long-term portfolios.Continue reading