DNA rearrangements are responsible for a variety of phenomena in unicellular organisms, such as bacterial or protozoan antigenic variation, yeast mating type switching, and bacterial nitrogen fixation. In multicellular organisms, however, DNA rearrangements do not seem to be a major contributor to cell determination; in fact the only mammalian tissue in which rearrangements play an identified vital role, the immune system, could be viewed as a collection of unicellular organisms. Perhaps in multicellular organisms there are sufficient alternative strategies to determine the fate of cell lineages, and the costs or risks of reshuffling the genome may be too high. Nevertheless, transposons are probably found in all organisms. Why?