IMGT Scientific chart

IMGT numbering of additional codons (or amino acids) and corresponding nucleotides

Additional codons (or amino acids) and corresponding nucleotides are identified by a number (the one of a neighbour reference codon) followed by a dot and number, or by a letter:

a dot and number, for expected additional codons (or amino acids)

C-DOMAIN and C-LIKE-DOMAIN (N-terminal end of stand A; AB, CD, DE and EF turns)
Examples:
- codon 1.1 (nucleotides 1.1, 2.1, 3.1)
- codon 1.2 (nucleotides 1.2, 2.2, 3.2)
- codon 1.3 (nucleotides 1.3, 2.3, 3.3)
- codon 45.1 (nucleotides 133.1, 134.1, 135.1)
- codon 45.2 (nucleotides 133.2, 134.2, 135.2)
- codon 84.1 (nucleotides 250.1, 251.1, 252.1)
- codon 84.2 (nucleotides 250.2, 251.2, 252.2)
- codon 85.1 (nucleotides 253.1, 254.1, 255.1)
CDR-IMGT of V-DOMAIN and V-LIKE-DOMAIN
- codon 60.1 (nucleotides 178.1, 179.1, 180.1)
- codon 112.1 (nucleotides 334.1, 335.1, 336.1)

a capital letter, for unusual insertions (or insertions that need to be underlined)

Examples:

FR-IMGT of V-DOMAIN and V-LIKE-DOMAIN
- codon 84A (nucleotides 250A, 251A, 252A)
  
  84A for 1 additional position
  
  84A, 84D for 2 additional positions
  
  84A, 84B, 84C for 3 additional positions
  
  84A, 84B, 84C, 84D for 4 additional positions
  
  The letter can be shown as a superscript for a greater readability (for instance in Tables of alleles)
- codon 84^A (nucleotides 250^A, 251^A, 252^A)

Note that

the "capital letter" and the "dot and number" are identical in the codon numbering and in the nucleotide numbering.
the codon number which is before the letter or the dot is multiplied by 3 to obtain the numbering of the third (most downstream) nucleotide of that codon.
The numbering of the second and first nucleotides are obtained by substracting 1 and 2, respectively.

84A	for 1 additional position
84A, 84D	for 2 additional positions
84A, 84B, 84C	for 3 additional positions
84A, 84B, 84C, 84D	for 4 additional positions