August 1, 2003. Retrieved August 9, 2015. ↑ Molecular Biology of the Cell, Fourth Edition, Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, Peter Walter ↑ ines.de/cope.cgi COPE database ↑ Campbell, Neil A. (1996) Biology Fourth Edition. Benjamin/Cummings Publishing, pp. 120-121 ISBN 0-8053-1940-9 ↑ Henze K, Martin W (2003).