El Salvador became the first country to formally implement CAFTA, which went into effect on March 1, 2006, when the Organization of American States (OAS) received signed copies of the treaty. On April 1, 2006, Honduras and Nicaragua fully implemented the agreement. On May 18, 2006, the Congress of Guatemala ratified CAFTA-DR, which went into effect on July 1, 2006. The Dominican Republic implemented the agreement on March 1, 2007. In a referendum on October 7, 2007, Costa Rica narrowly backed the free trade agreement, with 51.6% voting "Yes"; the agreement took effect on January 1, 2009.