Several schemes for antibody numbering exist to standardize the position of specific residues in the otherwise variable-length sequence.
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Chothia:
Ref (1)
1.
Chiu ML, Goulet DR, Teplyakov A, Gilliland GL. Antibody Structure and Function: The Basis for Engineering Therapeutics. Antibodies. 2019;8(4):55. Available from: https://doi.org/10.3390/antib8040055