The chemokines are a superfamily of 41 human ligands and at least 18 receptors (Zlotnik and Yoshie 2000). They have been divided into inflammatory and homeostatic chemokines depending on their expression patterns. The inflammatory chemokines are generally those that are produced by cells of the immune system upon activation. The homeostatic chemokines are those whose expression is constitutive and are generally produced by nonimmune cells. The chemokines by and large have been considered to be important in the regulation of leukocyte traffic in the body. This has been shown most dramatically in the case of a few chemokine receptors, including CCR7 and CXCR5. CCR7 has two ligands, CCL19 and CCL21, whose expression is eliminated in a natural mutant mouse called the paucity of lymph node T cells (PLT) mouse. This is a mouse that arose naturally and was identified in an animal facility in Japan (Gunn 1998). As its name indicates, this mouse does not have many T cells in its lymph nodes. Careful analysis of the gene expression in this mouse showed that both CCL19 and CCL21 are not expressed in the lymph nodes of these mice (Gunn 1998).