The IMGT numbering for coding regions is used for the description of mutations, allelic polymorphisms and structural data.
The L-REGION numbering starts with the first nucleotide of the INIT-CODON (initiation codon, usually 'atg').
The V-REGION numbering (for germline sequences)
starts with the first nucleotide of the first codon of the V-REGION. The encoded amino acid at position 1
is the one following the proteolytic cleavage site between the leader peptide and the V-REGION.
The delimitations of the FR-IMGT and CDR-IMGT regions and the amino acid positions are according to the IMGT unique numbering for V-REGIONs.
The J-REGION numbering (for germline sequences) starts with the first nucleotide of the first codon. The 1 (or 2) nucleotide(s) preceding the first codon (if present) are not numbered.
The V-J-REGION and V-D-J-REGION numbering (for rearranged genomic DNA or in cDNA sequences) starts with the first nucleotide of the first codon of the V-REGION, follows the IMGT unique numbering for V-DOMAIN.
Given that the C-REGION may be encoded by several exons and that these exons may encode different structural parts, two kinds of numbering are used: