Chemokine (C-X-C motif) ligand 9 (CXCL9), also known as monokine induced by gamma interferon (MIG), is a small cytokine belonging to the non-ELR (lacking a Glu-Leu-Arg motif in the N-terminal region) motif CXC chemokines. CXCL9 is one of the ligands to share its receptor, CXCR3, with other two CXC chemokine family members, CXCL10 and CXCL11, through different binding fashions. The rank order of binding affinity is CXCL11>CXCL10>CXCL9. Some studies have revealed that these ligands have redundant functions in vivo; other studies have demonstrated that the three CXCR3 ligands can collaborate and even compete with each other. CXCL9 is mainly produced by activated macrophages and some cancer cells. Functioning as a chemoattractant, CXCL9 attracts CXCR3-expressing cells (such as a subset of circulating T cells, B cells, and natural killer cells) into inflammatory sites. Like other chemokines, CXCL9 is also able to elicit intracellular calcium flux and activate PI3K and MAPK. It participates in multiple immune and inflammatory responses by affecting cell growth, movement, or activation.