ADAM20 was first cloned from a human testis library as a protein with homology to the fertilins (ADAMs 1, 2). ADAM20 message was seen predominantly in testis, but also at lower levels in placenta and B-cells. ADAM20 is closely related to ADAM21 (61% identical), ADAM29 (52% identical), and the testases (49-46% identical). ADAM20 contains the canonical HExxHxxxxxH zinc metalloproteinase motif, and the initial partial sequence of ADAM21 contained a non-functional catalytic site, making it tempting to speculate that ADAM20 and ADAM21 are orthologs to ADAMs 1&2. Later, a full-length ADAM21 sequence demonstrated a functional catalytic domain, but it may still be that the catalytically dead form of ADAM21 is expressed and functions as a fettilin. A member of the metalloproteinase family containing disintegrin-like domains (ADAMs) the function of ADAM20 is still poorly understood. Other ADAMs family members (ADAM10, ADAM17) have been more thoroughly studied, and are known to play key roles in inflammation, growth factor maturation and release, and a wide range of other functions. The full length ADAM20 sequence codes for a 726 amino acid protein, with a predicted mass is 81.711 kD. The sequence containing a Type-I transmembrane domain, metalloproteinase domain, cystein-rich domain, and a short cytoplasmic domain. Unlike many of the other ADAMs proteases, ADAM20 does not appear to contain a furin cleavage site.