Important role in the capacity of milk to transport calcium phosphate. Mammary gland specific. Secreted in milk. There are at least three different forms found in milk, with varying degrees of phosphorylation. These include form 10-P which is phosphorylated at ten sites that have not been determined, form 11-P which is phosphorylated at eleven sites and form 12-P which is phosphorylated at twelve sites. Variant A and dephosphorylated. Form 10-P. Form 11-P. Form 12-P. Variant A1. Variant A1d. Variant A2. Variant A2d. Belongs to the alpha-casein family.