Elafin is an epithelial proteinase inhibitor also known under various other names such as Skin-derived Anti leukoproteinase (SKALP) and Elastase-Specific Inhibitor (ESI). Elafin belongs to the Trappin gene family and was given the systematic name Trappin-2 in a recent classification. The Trappin family is defined by a N-terminal transglutaminase substrate domain and a C-terminal four disulphide core. Trappins have been suggested to play a role in the regulation of inflammation and in protection against tissue damage in stratified epithelia. Elafin is an inhibitor of leukocyte elastase and proteinase-3 and in addition it is a substrate for transglutaminases. The protein is constitutively expressed in various epithelia including those of hair follicles, oesophagus, vagina and oral cavity. Elafin is not present in normal human skin but is strongly induced during inflammation as in psoriasis and wound healing. The full-length protein is translated as a 12.3 kDa protein of 117 amino acids termed pre-elafin or Trappin-2. Cleavage of the signal peptide yields a mature protein with a molecular mass of 9.9 kDa. The 9.9 kDa secreted protein is the major form found in culture medium. In skin extracts a 6 kDa form comprising the 57 most C-terminal amino acids is present, which is the form of elafin originally described by Wiedow et al. In serum both the 9.9 and the 6 kDa form appear to be present. In urine only the short 6 kDa form is found. The Hycult biotech ELISA is not suitable to measure the short 6 kDa form. Elafin has been used as a quantitative marker for disease activity in severe psoriasis during treatment with cyclosporin. In serum/plasma of healthy individuals approximately 10-50 ng/ml elafin is present. Elafin shows a 10-fold increase in concentration during psoriasis. The determination of Elafin is not disturbed by the presence of the short 6 kDa form.