CCR5 and HIV infectious disease: CCR5, CCR2, CCR3, and CXCR4 are the genes encoding viral coreceptors, and the allelic variants and natural ligands (e.g., CCL3 transcripts and CXCL12/SDF-1) of these genes have been well studied in correlation with natural susceptibility or resistance to human immunodeficiency virus (HIV) infection [122].