Tryptases comprise a family of trypsin-like serine proteases, the peptidase family S1, and fall into two groups, α and β. β-tryptases appear to be the main isoenzymes expressed in mast cells, whereas α-tryptases predominate in basophils. Tryptase is unique in two respects: it is enzymatically active only as a heparin-stabilized tetramer, and it is resistant to all known endogenous proteinase inhibitors because of the unique arrangement of the active sites. Additionally, tryptase family genes have an intron immediately upstream of the initiator codon which separates the transcription initiation site from protein coding sequence, and this feature is characteristic of tryptases. β-tryptases existing in three isoforms (β1,β2,β3) are released in secretory granules, and have been implicated as mediators in the pathogenesis of asthma and other allergic and inflammatory disorders. It has been reported that β-tryptase selectively cleaves ASM-derived eotaxin and RANTES and abrogates their chemotactic activities.