Should Marriott terminate Starwood's relationship with Amex in favor of Marriott's co-branded program with JPMorgan Chase, Amex stands to lose its second largest co-branded partnership after Delta. Amex's Starwood portfolio of cards is worth 2 percent of Amex's 2015 billed business and 4 percent of 2015 loans. In an email to Starwood preferred guests on April 9, CEO Thomas Mangas and senior vice president of preferred guest Chris Holdren said Starwood didn't expect to launch a combined card program until 2018. "This means SPG will continue to run separately until then." The Starwood portfolio is not as large as the Costco portfolio, which represented about 8 percent of Amex's 2015 billed business and 19 percent of loans. But Starwood is "meaningful enough to hurt given other challenges faced by [Amex]," including its ongoing trial with the U.S. Department of Justice, according to a research note by RBC Capital Markets analyst Jason Arnold. Get the full story a Business Travel News