MCP-1/MCAF CCL2, also known as monocyte chemotactic and activating factor (MCAF), was initially purified independently by two groups based on its ability to chemoattract monocytes. Subsequent to its cloning and sequencing, it became evident that this protein is also identical to the product of the human JE gene. The JE gene, originally identified in mouse fibroblasts, is a plateletderived growth factor (PDGF)inducible gene. The human CCL2 cDNA encodes a 99 amino acid residue precursor protein with a 23 residue hydrophobic signal peptide that is cleaved to generate the 76 residue mature protein. Natural CCL2 is heterogeneous in size due to the addition of Olinked carbohydrates and sialic acid residues. In addition to fibroblastsI3/4 tumor cells, smooth muscle cells, endothelial cells, and mononuclear phagocytes can also produce CCL2 either constitutively or upon stimulation by various stimuli. CCL2 is a member of the beta (CC) subfamily of chemokines. Recently, the existence of MCP2 and MCP3 with 62% and 73% amino acid identity respectively, to CCL2 have been reported.