Protein S1-A7, also known as S1 calcium-binding protein A7, Psoriasin, S1A7, and PSOR1, is a secreted protein which belongs to theS-1 family. S1A7 was first isolated from skin involved by psoriasis, which can be induced in cultured squamous epithelial cells. S1A7 is 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. S1A7 plays a role in the pathogenesis of inflammatory skin disease, as a chemotactic factor for hematopoietic cells. It also plays a role in early stages of breast tumor progression in association with the development of the invasive phenotype.