Protists express variant surface antigens: of a mutlgene family always only one gene is expressed, as they are unicellular eukaryotes maybe this is what you want. you can choose Plasmodium, Trypanosoma, , or easy to culture free living species: Paramecium for instance.

I think that you will find good examples among olfactory receptors

Thanks guys! the olfactory system is indeed a good example and I'm looking for more examples

Maybe the examples mentioned here can be useful: http://www.nature.com/nrg/journal/v13/n6/full/nrg3239.html