May function in the secretion of milk-fat droplets. May act as a specific membrane-associated receptor for the association of cytoplasmic droplets with the apical plasma membrane (By similarity). Inhibits the proliferation of CD4 and CD8 T-cells activated by anti-CD3 antibodies, T-cell metabolism and IL2 and IFNG secretion (By similarity). Seems to associate with xanthine dehydrogenase/oxidase. N-glycosylated. Belongs to the immunoglobulin superfamily. BTN/MOG family.