Agmatidine, a modified cytidine in the anticodon of archaeal tRNA(Ile), base pairs with adenosine but not with guanosine.
Mandal D, Köhrer C, Su D, Russell SP, Krivos K, Castleberry CM, Blum P, Limbach PA, Söll D, RajBhandary UL. Proc Natl Acad Sci U S A. 2010 Feb 16; 107(7):2872-7. Epub 2010 Jan 26.