<prompt> Casein, the milk protein isNucleoproteinChromoproteinPhosphoproteinGlycoprotein  <response>: Phosphoproteins: These contain phosphorus. Casein of milk and vitellin of egg yolk are examples. The phosphoric acid is esterified to the hydroxyl groups of serine and threonine residues of proteins.Reference: Vasudevan 7th ed, pg 43 