Butyrophilin is a glycoprotein that is specifically expressed on the apical surface of mammary epithelial cells during lactation and becomes incorporated as an integral protein into the membrane of the milk fat globule during the budding and secretion of fat droplets into milk. BTNL2 (butyrophilin-like protein 2), also known as BTL-II, is a 455 amino acid single-pass type II membrane protein that acts as a negative regulator of T cell proliferation. Expressed in thymus, brain, kidney, heart, liver, small intestine, testis, ovary, pancreas and leukocytes, BTNL2 exists as at least six alternatively spliced isoforms, which are encoded by a gene located on human chromosome 6. Expression of BTNL2 isoform 3 is associated with susceptibility to sarcoidosis type 2 (SS2), an immune disorder that causes chronic inflammatory granulomatous lesions.