CCR5 and AIDS: It has been shown that the EC phenotype is also determined by host genetic elements such as acquired immunodeficiency syndrome (AIDS) restriction genes (ARGs), exemplified by the Δ32 deletion in the CCR5 gene, and specific expression of HLA class I alleles (e.g., B57:03, B81:01), both of which are associated with protection against the infection (International et al., 2010; McLaren et al., 2012).