CSN2

CSN2 is the gene symbol for beta-casein, one of the major proteins in milk and a member of the casein family, which also includes CSN1S1, CSN1S2, and CSN3. The gene is expressed in the mammary gland, and the beta-casein protein is secreted into milk, where it contributes to the formation of casein micelles along with other caseins and calcium phosphate.

Beta-casein provides nutritional value and texture to milk and plays a role in cheese-making through its involvement in micelle assembly and milk coagulation. The coding sequence of CSN2 varies among species, and there is notable genetic variation within cattle populations that affects the composition of milk.

Polymorphisms in CSN2 give rise to different beta-casein variants, most famously the A1 and A2 forms. A key difference is a single amino acid substitution at a residue corresponding to position 67, where A2 carries proline and A1 carries histidine. Digestive processing of milk containing the A1 variant can release BCM-7, a peptide that has been proposed to exert physiological effects, though health implications remain debated and contested in the scientific literature.

Breeding and dairy industries sometimes distinguish A2 milk products based on CSN2 alleles, aiming at specific milk properties and consumer preferences. CSN2 is a focus of nutrition and milk-protein research, with ongoing study of its evolution, expression, and functional roles in lactation across different species. In humans, CSN2 is studied as part of the broader casein gene family and its relevance to nutrition and infant digestion.