An orphon is a gene identified in a chromosomal location outside the main loci.
- IG orphons are localized outside the three main loci IGH, IGK and IGL .
- TR orphons are localized outside the four main loci TRA, TRB, TRG and TRD .
An orphon is either pseudogene or ORF. It is not functional.
By extension, an orphon is an instance of the 'gene' concept of IMGT-ONTOLOGY .
Orphons belonging to a same group and localized on a same chromosome constitute a 'chromosomal orphon set'.
The term orphon was introduced by Childs et al.  to refer to "genetic elements that have lost their families". It was first applied to solitary members of histone and ribosomal gene families PMID: 6784929.
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 Childs, G. et al., Cell, 23, 651-663 (1981) PMID: 6784929