All conceivable tetraplets which are supposed to occur in Ile-binding sites if one of its coding triplets, either AUU, AUC, AUA (Ile codons) or AAU, GAU, UAU (Ile anticodons) is already there. These tetraplets consist of codon and anticodon overlapping at the palindrome-dinucleotide. Marked by asterisk are tetraplets in which codon or anticodon represents a different (not Ile) amino acid. A. The tetraplets expected for palindrome AU at codons' 1-2 positions and anticodons 2-3 positions, respectively. Shown boxed is the tetraplet 5'-UAUU-3' which one can actually see in all Ile-binding sites of independently selexed RNA aptamers (184 of 185 cases). Remarkably, this highly conserved motif constitutes the Ile-specific internal loop that seems to be directly involved in function . B. The tetraplets expected for palindrome UA at codon 2-3 positions and anticodon 1-2 positions. Shown in oval is the ile-specific tetraplet 5'-AUAU-3' which one would expect to observe if the UA palindrome in overlapping anticodon UAU and codon AUA was favored by selection of RNA aptamers specifically binding Ile. Remarkably, in contrast to ubiquitous 5'-UAUU-3' in the group A (codon(1-2)/anticodon(2-3) tetraplets), none of Ile-binding sites contains the motif 5'-AUAU-3' in seemingly symmetric group B (codon(2-3)/anticodon(1-2) tetraplets).