Galectin-9 (gal-9) is a member of the galectin protein family which consists of glycan-binding proteins that share a conserved carbohydrate recognition domain (CRD) and a binding affinity for β-galactoside sugars. Initially, galectins were designated as S-type lectins since it was found that the first discovered galectin, now referred to as galectin-1, depended on thiols (reducing conditions) to be active. Since this was not the case for many other galectins that were discovered later, the designation S-type lectin is now considered outdated. The galectin family name was defined in 1994 and consisted at that time of four members, i.e., galectin-1 to −4 (Barondes et al. 1994; Leffler et al. 2004). Galectin-9 was added to the protein family in 1997 when it was described by three independent groups...