Butyrophilin is the major protein associated with fat droplets in the milk. It is a member of the immunoglobulin superfamily. It may have a cell surface receptor function. The human butyrophilin gene is localized in the major histocompatibility complex (MHC) class I region of 6p and may have arisen relatively recently in evolution by the shuffling of exons between 2 ancestral gene families

BTN in the residual plasma membrane of the fat globule envelope was concentrated in a network of ridges that were tightly apposed to the monolayer derived from the secretory granule. The ridges coincided with BTN labeling in the globule monolayer.