<unk>  is one of the five main nucleobases found in the nucleic acids DNA and RNA; the others being adenine, cytosine, thymine, and uracil.  With the formula C5H5N5O,<unk>  is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with conjugated double bonds. Being unsaturated, the bicyclic molecule is planar.  The<unk>  nucleoside is called<unk> nosine.
Guanine
Xanthine
Citrulline
Phenylalanine
Tyrosine
Asparagine
Arginine
Ethionine