Human CCL23 was initially identified in a cDNA library derived from human endothelial cells. CCL23 has six cysteine residues and posseses a separately encoded amino-terminal domain, characteristic of similar chemokines CCL6, CCL9, and CCL15. These four chemokines are weak agonists for CCR1; nevertheless, they can be proteolytically cleaved in the amino-terminus, resulting in forms with higher chemoattractant activity. Thus, NH2-terminus truncated forms of CCL23 have been detected in synovial fluid of patients with rheumatoid arthritis (RA). In vitro studies have shown that CCL23 can be processed by MMPs at the amino and carboxyl terminus. CCL23 (26-99) is the predominant stable form after using MMP1, 2, 3, 7, 8, or 12. CCL23 (30-99) is formed by MMP14, elastase, and proteolytic activity in arthritic synovial fluid. In addition, CCL23 has been detected in serum of patients with active RA, in conjunction with CXCL13, TNFSR9, M-CSF, and TNFalpha. High levels in serum of these cytokines correlate with joint inflammation and have been considered as biomarkers of active RA. Also, high levels of CCL23 have been detected in plasma of atherosclerotic patients. High expression of CCL23 has been found in skin of atopic dermatitis patients, and in mucosal epithelial cells and inflammatory cells in sinonasal tissues of patients with chronic rhinosinusitis (CRS).