The biological role of RNA is delimited by its possible reactions, which can be explored by selection. A comparison of selected RNAs that bind one ligand with those that bind two related ligands suggests that a single nucleotide substitution can expand binding specificity. An RNA site with dual (joint) specificity has adenine and cytosine bases whose pK(a)’s appear shifted upward, thereby mimicking an efficient general acid-base catalyst. The joint site also contains two conserved, looped arginine-coding triplets implicated in arginine site formation. Two selected joint RNAs are identical in some regions and distinct in others. The distinct regions, like some peptides, seem to function similarly without being similar in primary structure.