CCR5 and HIV-1 infection: CC-type chemokine receptor 5 (CCR5) delta32 is a naturally occurring 32 bp knockout deletion variant: homozygotes resistant to R5-HIV-1 infection (the principal infecting HIV-1 strain) lack the requisite HIV-1 entry co-receptor CCR5 on their lymphoid cells7,8, whereas heterozygotes express less than half the wild-type levels of CCR5 receptors, thus slowing HIV-1 replication, spread and pathogenesis9,10.