Important role in determination of the surface properties of the casein micelles. Mammary gland specific. Secreted in milk. There are at least three different forms found in milk, with varying degrees of phosphorylation. These include form 5-P which is phosphorylated at three sites, this form is present in low amounts, form 6-P which is phosphorylated at six sites, and form 7-P which is phosphorylated at seven sites. Variant A-Delta5. Belongs to the beta-casein family.