CCR5 Δ32 is a 32-base-pair deletion that introduces a premature stop codon into the CCR5 receptor locus, resulting in a nonfunctional receptor.[27][28]?CCR5 is required for M-tropic HIV-1 virus entry.[29]?Individuals homozygous for CCR5 Δ32 do not express functional CCR5 receptors on their cell surfaces and are resistant to HIV-1 infection, despite multiple high-risk exposures.[29]?Individuals heterozygous for the mutant allele have a greater than 50% reduction in functional CCR5 receptors on their cell surfaces due to dimerization between mutant and wild-type receptors that interferes with transport of CCR5 to the cell surface.[30]?Heterozygote carriers are resistant to HIV-1 infection relative to wild types and when infected, heterozygotes exhibit reduced viral loads and a 2-3-year-slower progression to AIDS relative to wild types.[27][29][31]?Heterozygosity for this mutant allele also has shown to improve one's virological response to anti-retroviral treatment.[32]?CCR5 Δ32 has an (heterozygote) allele frequency of 10% in Europe, and a homozygote frequency of 1%.