ADAM28 was first described in lymphocytes, also in Thymic Endothelial Cells (TECADAM), as well as in the epididymus. Later works describe two forms of ADAM28: ADAM28-S and ADAM28-L (short and long forms). The short form is a soluble, alternatively spliced form lacking the transmembrane and cytoplasmic domains. Message levels of the short form were found to be much lower than the long form in the tissues studied, but these may be tissue-specific differences. In humans, ADAM28 is reported in lymphocytes by some authors and to be absent from mature lymphocytes by others. The thyroid, lung, trachea, esophagus, and stomach have been shown to express human ADAM28 message. Knockout mice lacking B cells and T cells express ADAM28 message in the thyroid, lung, trachea, esophagus, and stomach. A member of the metalloproteinase family containing disintegrin-like domains (ADAMs) the functions of ADAM28 are still poorly understood. ADAM28 contains the canonical HExxHxxxxxH zinc metalloproteinase motif, and has been shown to be proteolytically active. 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 ADAM28 (ADAM28 Long) sequence codes for a 775 amino acid protein with a predicted mass is 87.2 kDa, and a pI of 6.50. The soluble ADAM28 (ADAM-28 Short) is a 540 amino acid sequence, with a predicted mass of 61.3 kDa, and a pI of 6.58. Other forms of ADAM28 have been reported with slightly different carboxyterminal regions. ADAM28 may play a role in the adhesive and proteolytic events that occur during lymphocyte emigration or may function in ectodomain shedding of lymphocyte surface target proteins, such as FASL and CD40L. It may be involved in sperm maturation. There are 2 isoforms of ADAM28: one short secreted and one long membrane bound.