Archaeosine is a 7-deazaguanine ribonucleoside having 7-formamidino-7-deazaguanine as the nucleobase. It is found in the majority of archaeal tRNAs specifically at position 15 of the dihydrouridine loop (D-loop), a position not modified in either eukaryotic or bacterial tRNA. It is functionally related to a 7-formamidino-7-deazaguanine.