 Some aminopeptidases are monomeric, and others are assemblies of relatively high mass (50 kDa) subunits.cDNA sequences are available for several aminopeptidases and a crystal structure of the open state of human endoplasmic reticulum Aminopeptidase 1 ERAP1 is presented here.Amino acid sequences determined directly or deduced from cDNAs indicate some amino acid sequence homologies in organisms as diverse as Escherichia coli and mammals, particularly in catalytically important residues or in residues involved in metal ion binding.