The ERAP1 gene provides instructions for making a protein called endoplasmic reticulum aminopeptidase 1. As its name suggests, this protein is active in a cellular structure called the endoplasmic reticulum, which is involved in protein processing and transport. This protein is an aminopeptidase, which is an enzyme that cuts (cleaves) other proteins into smaller fragments called peptides.~Endoplasmic reticulum aminopeptidase 1 has two major functions, both of which are important for normal immune system function. First, the protein cleaves several other proteins called cytokine receptors on the surface of cells. Cleaving these receptors reduces their ability to transmit chemical signals into the cell, which affects the process of inflammation.~Second, endoplasmic reticulum aminopeptidase 1 cleaves many types of proteins into small peptides that can be recognized by the immune system. These peptides are exported to the cell surface, where they attach to major histocompatibility complex (MHC) class I proteins. MHC class I proteins display the peptides to the immune system. If the immune system recognizes the peptides as foreign (such as viral or bacterial peptides), it responds by triggering the infected cell to self-destruct.~While the protein is involved in the normal functioning of the immune system, it plays a particular role in protecting the body against the development of autoimmune disorders and cancer.