The galectins, formerly known as S-type lectins, are a family of β-galactoside-binding proteins implicated in modulating cell-cell and cell-matrix interactions. Galectin-9 (LGALS9) is a tandem repeat-type member of the galectin family. It has three isoforms (named galectin-9L, galectin-9M, and galectin-9S): long type of 355 amino acids, medium type of 323 amino acids, and short type of 311 amino acids. Galectin-9 is ubiquitously expressed in a variety of tissues, including lymph nodes and spleen, and overexpressed in Hodgkin disease tissue. It is involved in chemoattraction, apoptosis, and the regulation of cell differentiation and has anti-allergic effects. It has been reported that galectin-9 is a ligand for Tim-3, through which galectin-9 can induce T-helper type 1 lymphocyte (Th1) death.