A three-ring system that is the active group of
flavin cofactors. For those who are interested:
C1(=O)NC(=O)C2=Nc3cc(C)c(C)cc3N(R)C2=N1
It is an ancient molecule that looks strangely similar to a nucleotide (and is one, essentially as FMN). Its synthesis is from GTP - as can be seen from the nitrogen hetrocyclic rings.