Human psoriasin was first identified as an over-expressed secreted protein in psoriatic skin. Psoriasin also called S100A7, is distributed in the cytoplasm of keratinocytes in normal human epidermis and is present at the cell periphery in terminally differentiated keratinocytes. The peripheral distribution observed in differentiated cells may be important, since, under some conditions, S100A7 may be released from keratinocytes. Several S100 proteins are thought to be secreted. Indeed, S100A7 has been shown to function as a chemotactic agent and as a cytokine, and to attract CD4+ lymphocytes and neutrophils. Originally characterized as a marker of psoriasis, S100A7 overexpression is seen in many epidermal inflammatory diseases, including atopic dermatitis, mycosis fungoides, Darier's disease and inflammatory lichen sclerosus at atrophicus. The high level of expression in active psoriatic lesions has prompted investigators to suggest that S100A7 may have a chemotactic role in psoriasis.