The principal protein fraction of cows' milk. It accounts for about 80% of the protein content and is present in concentrations of 2.5–3.2%. The term casein, derived from the Latin word for cheese, caseus, was introduced into the scientific literature early in the nineteenth century to describe the primary protein of milk. Casein is a mixed complex of phosphoproteins existing in milk as colloidally dispersed micelles 50 to 600 nanometers in diameter. The variable-sized micelles are assembled from spherical subunits of nearly uniform diameter (10 to 20 nm), containing 25–30 casein molecules, and from small amounts of calcium phosphate which, together with serum ionic calcium, plays a significant role in micellar structure. Critical to the stability and size distribution of the micellar state is a calcium-insensitive phosphoprotein component which functions as a protective colloid to the system. When hydrolyzed by rennin (chymosin) as in the cheese-making process, it loses its hydrophilic, carboxy-terminal segment, predisposing the altered micellar system to the aggregation activity of calcium ions and subsequent clot formation. See also: Colloid; Micelle