Amino acid pairs coded by complementary codons. Two optimal (perfect) and six suboptimal (partial) codon complementarity situations (codon codes) are listed. In the perfect complementarity situation a codon (AUG), which is transcribed from the sense (pos) DNA strand in the 5'>3' direction, is complemented with the UAC codon that is transcribed from the antisense (neg) DNA strand in complementary (C-123) or reverse-complementary (RC-321) orientations. In the suboptimal codon codes, one codon residue is undefined (X) and may or may not complemented in the corresponding codon on the negative strand in that residue position. Translation of the codon and its complementary pair will result in different amino acid pairs, depending on the codon pattern. This is illustrated in examples where the undefined X residues uniformly replaced by A in the positive and U in the negative strands. For example, the meaning of 5'-AAG-3'/3'-UUC-5' (from the D_1X3/RC_3X1 codon code pattern) is that this codon pair will be translated into the amino acids Lys (K) and Leu (L) and will result in K><L residue pairs. Letter -P at the end of a codon pattern indicates the presence (-P), in contrast to the non-presence (-N), of that particular codon code to determine a specific amino acid co-location in a concrete protein structure (this is used in Figure 6).