Framework regions (FR-IMGT)
The Framework Regions (FR) and the Complementarity Determining Regions (CDR) are parts of the variable regions of the Immunoglobulin and T cell Receptor chains. The IMGT unique numbering  has allowed to redefine the limits of the Framework Regions and Complementarity Determining Regions, designated as FR-IMGT and CDR-IMGT, respectively in IMGT (IMGT Scientific chart).
The lengths of the FR-IMGT and CDR-IMGT are in themselves crucial information which characterize variable regions belonging to a group, a subgroup and/or a gene [2,3]. Tables of the FR-IMGT and CDR-IMGT lengths are available in the IMGT Repertoire. Framework amino acids (and codons) located at the same position in different sequences can be compared without requiring sequence alignments.
 Lefranc, M.-P., The Immunologist, 7, 132-136 (1999) LIGM:217.
 Lefranc, M.-P. et al., Nucleic Acids Res., 27, 209-212 (1999) PMID: 9847182, LIGM:209.
 Scaviner, D. et al., Exp. Clin. Immunogenet., 16, 234-240 (1999) PMID: 10575277, LIGM:218