The immune system uses DNA recombination to activate one particular gene by bringing promoter and enhancer regions into close proximity [1].
To explain the mutually exclusive expression, gene conversion is another possible mechanism. In yeast mating-type switches [2] or in Trypanosoma antigenicity changes [3] the chosen copy from a group of related genes is transferred to an expression cassette located at a distant site.
The mutually exclusive regulation of olfactory receptors remains to be determined [4].