Lactoperoxidase is an antibacterial agent in cow milk. The heme protein lactoperoxidase (LPO), also referred to as salivary peroxidase (SPO), is an oxidoreductase secreted into milk. LPO, a 712 amino acid protein, belongs to the XPO subfamily of the peroxidase family. It is expressed in mammary and salivary glands, and in the presence of H2O2, LPO acts as a catalyst for the oxidation of many phenols and aromatic amines. It is crucial for protecting the lactating mammary gland and intestinal tract of newborn infants against microorganisms. LPO binds one calcium ion per heterodimer and one heme B (iron-protoporphyrin IX) group covalently per heterodimer. The LPO gene, which spans 28 kb, is similar in gene organization and sequence to the peroxidase genes MPO and EPX, suggesting the possibility that these genes evolved from a common ancestral gene. The LPO and MPO genes are arranged in a tail-to-tail manner on chromosome 17q23.1.