The 'IMGT unique numbering' concept is part of the 'NUMEROTATION' concept of IMGT-ONTOLOGY 
The IMGT unique numbering for all IG and TR V-REGIONs of all species relies on the high conservation of the structure of the variable region [1-6]. This numbering, set up after aligning more than 5 000 sequences, takes into account and combines the definition of the framework (FR) and complementarity determining regions (CDR) , structural data from X-ray diffraction studies , and the characterization of the hypervariable loops . The delimitations of the FR-IMGT and CDR-IMGT regions have been defined and Correspondence between the IMGT unique numbering and the other numberings has been established .
The IMGT unique numbering has many advantages:
By facilitating comparisons between the sequences and the descriptions of alleles and mutations, the IMGT unique numbering represents a big step forward in the analysis of the IG and TR sequences of all species. Moreover, it gives in-sight in the structural configuration of the variable domain and opens interesting views on the evolution of the sequences of the V-set, since this numbering has been applied with success to all the sequences belonging to the V-set of the immunoglobulin superfamily, including nonrearranging sequences in vertebrates (CD4, Xenopus CTX,...) and in invertebrates (Drosophila Amalgam, Drosophila Fasciclin II, etc.) [1, 2, 9].
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