In Salmonella, the production of surface antigens is regulated by a simple inversion switch in which a 1000 bp promoter sequence "flips" to activate one of two genes.  This antigenic switching enables Salmonella to evade host immune response by producing antigens which are noncompatible with activated antibodies.


 
 
 
 
 
 
 
 
 
 
 

Trypanosoma has several thousand genes responsible for variable surface glycoproteins (VSGs).  When antigen switching takes place, a copy of a VSG gene is transposed and takes the place of a previously active gene, under the control of several promotors present at the active site, so only one surface antigen is produced at any one time.


 
 
 
 
 
 
 
 
 
 
 

In Saccharomyces cerevisiae, two mating types, a and a, which can fuse to produce a diploid zygote.  While in the haploid (N) generation, either of the two mating types can divide mitotically to produce a mixed population of both mating types.  This is controlled by a "mating-type" switch involving both a copy of one of the two genes, and inversion switching.  When nutrients are depleted, the diploid cell can undergo meiosis to form spores.