All twelve signed the TPP on 4 February 2016. The agreement would have entered into force after ratification by all signatories, if this had occurred within two years. If the agreement had not been ratified by all before 4 February 2018, it would have entered into force after ratification by at least 6 states which together have a GDP of more than 85% of the GDP of all signatories. The withdrawal of the United States from the agreement in January 2017 effectively ended any prospect of the agreement entering into force. In response the remaining parties successfully negotiated a new version of the agreement that lacked the 85% GDP threshold, the CPTPP, which entered into force in December 2018. Withdrawn member On 23 January 2017, US President Donald Trump signed a presidential memorandum to withdraw the United States' signature from the agreement, making its ratification as it was in February 2016 virtually impossible. On 13 April 2018, Trump said the United States may rejoin the Trans-Pacific Partnership. Comparison of initial signatories to the agreement Potential members APEC members would have been able to accede to the TPP, as would any other jurisdiction to which existing TPP members would have agreed. After an application for membership is received, a commission of parties to the treaty would negotiate conditions for accession. South Korea did not participate in the 2006 agreement, but showed interest in entering the TPP, and was invited to the TPP negotiating rounds in December 2010 by the U.S. after the successful conclusion of its Free trade agreement between the United States of America and the Republic of Korea. South Korea already had bilateral trade agreements with some TPP members, but areas such as vehicle manufacturing and agriculture still needed to be agreed upon, making further multilateral TPP negotiations somewhat complicated. Other countries that were interested in TPP membership include Taiwan, the Philippines, and Colombia as of 2010; Thailand as of 2012; and Indonesia, Bangladesh, and India as of 2013. According to law professor Edmund Sim in 2013, many of these countries would need to change their protectionist trade policies in order to join the TPP. The largest economy in the Pacific Rim not involved in the negotiations was China. According to the Brookings Institution in 2013, the most fundamental challenge for the TPP project regarding China was that "it may not constitute a powerful enough enticement to propel China