Orphons are a class of dispersed, solitary genetic elements of the genome derived from tandem multigene families. These displaced elements arise from genes that do or do not code for proteins, including those of histones in the sea urchin Lytechinuspictus; ribosomal genes in Saccharomyces cerevisiae; ribosomal genes and H3 histone genes in Drosophila melanogaster (1); the amylase multigene family of mice (2); the spliced-leader small RNA gene family of Trypanosoma brucei (3) and of the nematode Angiostrongylus cantonensis (4); and the human 28 S ribosomal DNA (5).
An interesting case is that of the human immunoglobulin V kappa gene regions. They have been transposed during evolution from the site of the kappa locus on chromosome 2 to chromosomes 1, 22, and other chromosomes. These orphons are very similar and may have derived from a single ancestral gene. The DNA sequences at the junction of chromosome 22 and other orphon regions are direct and inverted repeats and in one case an Alu sequence repeat. These unusual features may have predisposed the orphon regions to transposing by serving as target sites for enzymes involved in recombination (6).
