Both HLA-B27 and HLA-57 have been identified in terms both HIV and HCV infection. The mechanisms for this joint efficacy could be manifold. First, it is possible that both HLA alleles could be in linkage disequilibrium with other genes in the major histocompatibility complex which are more directly responsible for their effects. Second, it is possible that both HLA molecules select for epitopes that may have an influence on viral fitness. Third, it is possible that such associations are simply related to the prevalence of a particular gene in a population because it is likely that in any given population there will be less exposure to and consequently less selection against, rare alleles, particularly in population where the prevalence of the condition has increased recently. It is worth noting that there are also striking differences between HIV and HCV in HLA types association with protection.