Fannie Mae’s largest seller-servicers are beginning to issue mortgage-backed securities in so-called quarter coupons to help reduce their exposure to mortgage servicing right risk. Washington Mutual says it has signed commitments to issue $10 billion of quarter-coupon Fannie Mae MBSs so far this year, or about 12 percent of the company’s total issuance with the government-sponsored enterprise. In 2002, WaMu did only $1.5 billion in quarter-coupon Fannie MBSs, all of it coming in