Alpha-lactalbumin is the B protein of lactose synthetase secreted by the mammary epithelial cells. It is a potent Ca2+-elevating and apoptosis-inducing agent with broad, yet selective, cytotoxic activity. Multimeric ?lactalbumin has been shown to kill all transformed, embryonic and lymphoid cells tested, but not mature epithelial elements. This suggests that milk contributes to mucosal immunity not only by furnishing antimicrobial molecules but also by policing the function of lymphocytes and epithelium. ?lactalbumin may be helpful in discovering the site of origin of metastatic breast tumors. Human lactalbumin contains 123 amino acid residues. Comparison of the 5' flanking sequences of the two Alpha-lactalbumin genes with those of five casein genes reveals the presence of a highly conserved region extending from position -140 to -110 in all seven sequences examined, suggesting a possible regulatory role in the hormonal control or tissue-specific expression of milk protein genes in the mammary gland.