ADAM18 was first described as tMDC III (testis-specific Metalloproteinase-like disintergin-like cysteine-rich protein-III). ADAM18 is a member of the ADAMs family (A Disintegrin And Metalloproteinase), but does not contain the canonical HExxHxxxxH zinc-binding metalloproteinase catalytic site. The domain structure of the fill-length ADAM18 includes a signal sequence, propeptide domain, metalloproteinase-like domain, disintegrin-like domain, cys-rich domain, EGF-like domain, a short spacer region, then the transmembrane domain and a cytoplasmic domain. Like many of the reproductive-specific ADAMS, ADAM18 plays a nonenzymatic role, or (as is the case for ADAMs 1 & 2 (fertilin alpha and beta)), the protein acts in concert with a proteolytically active ADAM to process proteins. Little is known about interactions between ADAM18 and other ADAMs. To date, 4 different sequences for human ADAM18 are published; 739, 715, 182 and 153 amino acids in length.