CCL25, also known as TECK, is a CC chemokine that regulates the trafficking of lymphocytes in the thymus and small intestine. Mature human CCL25 shares 40% amino acid sequence identity with mouse and rat CCL25. CCL25 is produced by stromal cells in the thymus and epithelial cells of the small intestine, particularly the jejunum and ileum. It binds to and induces chemoattraction through CCR9, and both human and mouse proteins act on human CCR9. CCR9 is expressed on immature pre-T cells and thymocytes. In cancer, functional CCR9 mediates the metastasis of melanoma cells to the small intestine, contributes to the CCL25-dependent migration and invasion of some breast carcinomas, and attracts mesenchymal stromal cells to CCL25-expressing multiple myelomas. CCL25 contributes to the severity of chronic inflammation in rheumatoid arthritis where it attracts CCR9+ monocytes and macrophages, in endometriosis where it promotes the invasiveness of stromal cells, and in atherosclerosis where it contributes to the accumulation of CCR9+ macrophages in arterial plaques.