Lectin specific for galactose (Gal) and its derivatives. Induces apoptosis. Has cytotoxic activity against several human cancer cell lines. Is less cytotoxic to normal human cells. Disulfide-linked dimer of A and B chains. Glycosylated. High-mannose type oligosaccharides. Cytotoxic activity repressed by addition of Gal and its derivatives. Melibiose preferentially protects normal cells from apoptosis as compared with transformed cells. The inhibitory concentration 50% (IC50) is 0.8 ng/ml for transformed cells and 10 ng/ml for normal cells.