Nucleobase
They include cytosine, guanine, adenine, thymine in (DNA), uracil in (RNA) and xanthine and hypoxanthine (mutated forms of guanine and adenine). These are abbreviated as C, G, A, T, U, X and HX respectively. They are usually simply called bases in genetics. Because A, G, C and T appear in the DNA, these molecules are called DNA-bases; A, G, C and U are called RNA-bases.