HCP5 is a region present on chromosome 6p21.3 that is characterized by multiple duplicated gene families. HCP5 (HLA class I histocompatibility antigen protein P5), also known as P5-1, is a 132 amino acid protein that is encoded by a gene mapping to human chromosome 6p21.33. Localizing within the MHC class I region, HCP5 is not structurally related to other MHC class I genes, but does have high sequence similarity with HERV-L and HERV-16. HCP5 also has high sequence homology to retroviral Pol genes, making it a possible candidate for interaction with HIV-1 through an antisense mechanism that prevents retrovirus transcription. Single-nucleotide polymorphisms (SNPs) to the region of the HCP5 gene that corresponds with HLA-B has been linked to a lower HIV-1 viral set point. HCP5 is highly expressed in lymphoid tissues, spleen and activated lymphocytes, as well as B-cell and natural killer (NK) cell lines.