C-C Motif Chemokine 21a (CCL21a) is a small secreted cytokine that belongs to the intercrine beta (CCchemokine) family. Mouse CCL21 cDNA encodes a 133 amino acid residue protein with a 23 residue signalpeptide that is cleaved to generate the 110 residue mature protein. Mouse CCL21 has three forms whileCCL21a has Ser-65. CCL21 elicits its effects by binding to a cell surface chemokine receptor known as CCR7 andCXCR3. Mouse CCL21 inhibits hemopoiesis and stimulates chemotaxis. It has chemotactic function in vitro forthymocytes and activated T-cells, but not for B-cells, macrophages, or neutrophils. Mouse CCL21 showspreferential activity towards naive T-cells and may play a role in mediating homing of lymphocytes tosecondary lymphoid organs.