In June 2013 the World Heritage Committee added Volume One of Das Kapital to the Memory of the World register. This decision made De Gruyter the world's only publisher with two influential thinkers in its catalog who works are protected by the UNESCO register. The second thinker is Gottfried Wilhelm Leibniz, whose Letters have been in the register since 2007. The Letters, which were written between 1702 and 1707 to the Astronomer Gottfried Kirch, were published for the first time by De Gruyter in 1900. The Karl Marx & Friedrich Engels Complete Edition (MEGA), which includes Das Kapital, has been published since 1990 by Akademie, which joined the De Gruyter Group in January 2013.