α-Lactalbumin is an important whey protein in cow′s milk, and is also present in the milk of many other mammalian species. In primates, α-lactalbumin expression is upregulated in response to the hormone prolactin and increases the production of lactose. α-Lactalbumin forms the regulatory subunit of the lactose synthase (LS) heterodimer and β-1,4-galactosyltransferase (β4Gal-T1) forms the catalytic component. Together, these proteins enable LS to produce lactose by transferring galactose moieties to glucose. As a monomer, α-lactalbumin strongly binds calcium and zinc ions and may possess bactericidal or antitumor activity. When formed into a complex with Gal-T1, a galactosyltransferase, α-lactalbumin, enhances the enzyme′s affinity for glucose by about 1000 times, and inhibits the ability to polymerize multiple galactose units. This gives rise to a pathway for forming lactose by converting Gal-TI to Lactose synthase.