A group is a set of genes which share the same 'gene type' and participate potentially to the synthesis of a polypeptide of the same 'chain type'. By extension, a group includes the related pseudogenes and orphons. The 'group' concept is independent from the species.
The 'group' concept is part of the 'CLASSIFICATION' concept of IMGT-ONTOLOGY .
In IMGT, groups are defined for the immunoglobulins (IG), T cell receptors (TR) and major histocompatibility complex (MHC) molecules. In order to manage the related proteins of the immune system (RPI) belonging to the immunoglobulin superfamily (IgSF) and to the MHC superfamily (MhcSF) in the IMGT databases and Web resources, additional groups have been defined.
Immunoglobulins and T cell receptors
A group is designated by four capital letters, the three first letters for the 'locus' to which belong the genes, and the fourth one for the 'gene type' (V, D, J or C) [2,3]. By extension, a group includes genes of the related chromosomal orphon sets.
There are 24 instances of the 'group' concept for the IG and TR (10 for the IG and 14 for the TR):
There are three instances of the 'group' concept for the genes which encode the PreB and PreT receptor chains:
- PREBC: PreB-ClambdaLike group
- PREBV: PreB-VLike group
- PRETA: PreT-Alpha group
Major histocompatibility complex
A group is designated by the three capital letters MHC, followed by a hyphen and the MHC type (-Ia, -Ib, -IIa, -IIb) and, for the MHC-II, by a hyphen followed by the 'gene type' (-A or -B). There are six main instances of the 'group' concept for the MHC, which correspond to the group of genes encoding the MHC-Ia-Alpha, MHC-Ib-Alpha, MHC-IIa-Alpha, MHC-IIa-Beta chains, MHC-IIb-Alpha and MHC-IIb-Beta chains, respectively
By extension, the B2M genes constitute a seventh instance.
IgSF other than IG and TR
Genes which encode chains of IgSF proteins other than IG and TR belong to a unique group defined as 'IgSF RPI'.
MhcSF other than MHC
Genes which encode chains of MhcSF proteins other than MHC belong to a unique group defined as 'MhcSF RPI'. As these genes encode MHC-I-like-Alpha.
 Giudicelli, V. and Lefranc, M.-P., Bioinformatics, 15, 1047-1054 (1999) PMID: 10745995, LIGM:221
 Lefranc, M.-P. and Lefranc, G., The Immunoglobulin FactsBook, Academic Press, 458 pages (2001) ISBN: 012441351X
 Lefranc, M.-P. and Lefranc, G., The T cell receptor FactsBook, Academic Press, 398 pages (2001) ISBN: 0124413528