^Ambrogelly, A.; Palioura, S.; Söll, D. Natural expansion of the genetic code. Nature Chemical Biology. 2007, 3 (1): 29–35. PMID 17173027. doi:10.1038/nchembio847.
^Böck, A.; Forchhammer, K.; Heider, J.; Baron, C. Selenoprotein synthesis: An expansion of the genetic code. Trends in Biochemical Sciences. 1991, 16 (12): 463–467. PMID 1838215. doi:10.1016/0968-0004(91)90180-4.
^Théobald-Dietrich, A.; Giegé, R.; Rudinger-Thirion, J. L. Evidence for the existence in mRNAs of a hairpin element responsible for ribosome dependent pyrrolysine insertion into proteins. Biochimie. 2005, 87 (9–10): 813–817. PMID 16164991. doi:10.1016/j.biochi.2005.03.006.
^Lu, Y.; Freeland, S. On the evolution of the standard amino-acid alphabet. Genome Biology. 2006, 7 (1): 102. PMC 1431706. PMID 16515719. doi:10.1186/gb-2006-7-1-102.