Galectin is widely distributed in nematodes, insects, porifers, as well as vertebrates, and it has also been found to be present in true fungi. Galectin does not just occur in the cytoplasm. It is also present in the nucleus, on the cell surface, in the extracellular matrix, etc., and it is thought to be involved in many biological phenomena, including cell development, cell differentiation, cell morphogenesis, tumor metastasis, cell death, and RNA splicing. Galectin-3 is a β-galactoside-binding protein that has been named IgE-binding protein, CBP35, CBP30, Mac-2, L-29, L-31, L-34, etc., and structurally it is a chimera-type lectin composed of a sugar-chain-binding domain (galectin domain) and a non-lectin domain. Its biological function is still uncertain, but many studies that elucidate its function have been performed, and as a result participation of galectin-3 has been demonstrated in the biological phenomena of cell growth, adhesion, metastasis, and apoptosis. For example, a positive correlation has been shown between galectin-3 expression and the degree of malignant transformation in certain types of cell lines. A positive correlation has also recently been shown between galectin-3 expression and degree of malignancy in certain types of malignant tumors, and measurement of galectin-3 is expected to possibly serve as an index of degree of tumor malignancy.