Psoriasin (S100A7) is a relatively new member of the S100 gene family that is located within the S100 gene cluster on chromosome 1q21 and shares the typical calcium binding domains that define this family of proteins. It was first identified as a 11.4 kDa cytoplasmic and secreted protein isolated from skin involved by psoriasis, which can be induced in cultured squamous epithelial cells. It is now known to be expressed by both normal cultured and malignant keratinocytes and malignant breast epithelial cells within ductal carcinoma in situ, suggesting an association with abnormal pathways of differentiation. Current evidence supports a role in the pathogenesis of inflammatory skin disease, as a chemotactic factor for hematopoietic cells, and a role in early stages of breast tumor progression in association with the development of the invasive phenotype. While therapeutic potential as a target for modulation of the inflammatory response in psoriasis awaits further studies, potential clinical applications already include a role as a detection marker for squamous cell carcinoma and a diagnostic marker to distinguish in situ from invasive breast cancer cells.