Galectins are a class of proteins that bind specifically to β-galactoside sugars. There have been 15 galectins discovered in mammals, encoded by the LGALS genes. Only galectin-1, -2, -3, -4, -7, -8, -9, -10 and -12 have been identified in humans. Galectin-13, also known as placental protein 13, is a placenta-specific galectin that induces the apoptosis of T lymphocytes, which may reduce the danger of maternal immune attacks on the fetal semiallograft during the long gestation of anthropoid primates.