Important role in determination of the surface properties of the casein micelles. Mammary gland specific. Secreted in milk. There are at least five different forms found in milk, with varying degrees of phosphorylation. These include form 3-P which is phosphorylated at three sites that have not been determined, this form is present in very low amounts, form 4-P which is phosphorylated at four sites, form 5-P which is phosphorylated at five sites, form 6-P which is phosphorylated at six sites, and form 7-P which is phosphorylated at seven sites. Spontaneous deamidation of Asn-150 produces aspartate or isoaspartate. Dephosphorylated. The measured range is 16-241. Dephosphorylated. The measured range is 16-109. Form 4-P. The measured range is 16-109. Form 5-P. The measured range is 16-109. Form 6-P. The measured range is 16-109. Form 7-P. The measured range is 16-109. Accounts for 4% of total casein. Belongs to the beta-casein family.