During relapsing fever, the etiologic spirochetes employ antigenic variation to avoid early immune clearance. In Borrelia hermsii, an agent of tick-borne relapsing fever, a switch in serotype is associated with a change in the predominant protein on the surface of the cells. The variable major proteins differ from one another in primary structure along their lengths. Genes encoding the different variable major proteins can be found in both "silent" and "active" environments. At the active site the gene is expressed; at the silent site the gene appears to be in storage. In variant cells, which express the new serotype-specific surface protein in a population, a gene from a silent site has replaced the previously sitting gene at the active site through transposition.