The endoplasmic reticulum (ER) aminopeptidase 1 (ERAP1) is a 120 kDa protein localized to the lumen of the ER, which removes NH2-terminal residues from many antigenic precursors for MHC class I peptide presentation. Peptides that are presented by MHC class I on the surface of a cell must be 8-11 residues long, and ERAP1 specifically trims peptides of 9 amino acids or more. ERAP1 is also induced by interferon- gamma. The gene encoding human ERAP1 maps to chromosome 5q15. ERAP1 has previously been characterized as adipocyte-derived leucine aminopeptidase (A-LAP), puromycin-insensitive leucine-specific aminopeptidase (PILS-AP) and aminopeptidase regulator of TNFR1 shedding (ARTS-1). A-LAP is thought to inactivate several bioactive peptides, including angiotensin II and, subsequently, may be involved in the regulation of blood pressure. PILS-AP is described as playing a role in angiogenesis by regulating the proliferation and migration of endothelial cells, and ARTS-1 is characterized as a TNFR1 binding protein that promotes TNFR1 shedding. Further research will be necessary to fully elucidate the functions of this protein.