This family of genes is responsible in Plasmodium falciparum for the antigenic variation protein along the surface of the infected erythrocyte. These genes (often more than 50) are scattered over most of the malarial chromosomes. Similar genes (called vir genes) exist in P. vivax and others (called SICAvar) are described in P. knowlesi. The switching rate from one set of antigens to another one is thought to be high; e.g., apparently two percent of P. falciparum stages switched to a new antigen in every erythrocytic cycle. Thus the variations seem non-limited.