CCR5 and infection: Host factors can typically be divided into three categories (Chatterjee, 2010): 1) genes encoding cell-surface receptors or ligands for these proteins, including the classic CCR5 32-bp deletion; 2) genes within the human leukocyte antigens (HLA) that regulate host responses to infection; and 3) other cytokine and immune response genes, including the homozygosity at position − 308 in the TNF-alpha promoter.