C-C Motif Chemokine 21a (CCL21a) is a small secreted cytokine that belongs to the intercrine beta (CC chemokine) family. Mouse CCL21 cDNA encodes a 133 amino acid residue protein with a 23 residue signal peptide that is cleaved to generate the 110 residue mature protein. Mouse CCL21 has three forms while CCL21a has Ser-65. CCL21 elicits its effects by binding to a cell surface chemokine receptor known as CCR7 and CXCR3. Mouse CCL21 inhibits hemopoiesis and stimulates chemotaxis. It has chemotactic function in vitro for thymocytes and activated T-cells, but not for B-cells, macrophages, or neutrophils. Mouse CCL21 shows preferential activity towards naive T-cells and may play a role in mediating homing of lymphocytes to secondary lymphoid organs.