The CDR 3 Region

The CDR 3 residues are divided symmetrically between cysteine residue 106 and the first glycine of the framework 4 beta bulge in position 140 (Kabat L99, H104, A107, B109, C109 and D107, undefined in IMGT), leaving ample space for long CDR 3 sequences and centering the CDR 3 gap around position 123. The number of residues allowed for the CDR 3 length variability may seem excessive, but CDR3 loops close to that length can be found in the Kabat database.  For even longer CDR 3s insertion nomenclature wil have to be used.