Analysis of the protein patterns of normal and psoriatic noncultured unfractionated keratinocytes has revealed several low-molecular-weight proteins that are highly up-regulated in psoriatic epidermis. Here, we have cloned and sequenced the cDNA (clone 1085) for one of these proteins that we have termed psoriasin. The deduced sequence predicted a protein of molecular weight of 11,457 daltons and a pI of 6.77. The protein co-migrated with psoriasin as determined by two-dimensional (2D) gel analysis of [35S]-methionine-labeled proteins expressed by RK13 cells transfected with clone 1085 using the vaccinia virus expression system. Analysis of the predicted sequence revealed a potential calcium-binding sequence of the EF-hand type, as well as the absence of a signal sequence at its amino terminal. Psoriasin is not related to other proteins that migrate closely in 2D gels (MRP 14, also known as calgranulin B, L1 and calprotectin; MRP 8, or calgranulin A and cystatin A or stefin A), and bears no significant sequence homology with any other protein of known primary structure. Increased expression of psoriasin mRNA in psoriatic keratinocytes was confirmed by Northern blotting and in situ hybridization. Psoriasin showed a restricted occurrence in fetal human tissues as determined by 2D gel electrophoresis. Of 21 tissues analyzed, only ear, skin, and tongue showed significant levels of this protein. Psoriasin was not detected in normal human fibroblasts, lymphocytes, endothelial cells and transformed epithelial cells of keratinocyte origin. Granulocyte extracts contained this protein suggesting that its overexpression by psoriatic keratinocytes may be linked to the inflammatory stimuli.