Brefeldin A is a lactone antibiotic produced by fungal organisms such as Eupenicillium brefeldianum. Brefeldin A inhibits protein transport from the endoplasmic reticulum to the Golgi apparatus indirectly by preventing formation of COPI-mediated transport vesicles. Brefeldin A was initially isolated as an anti-viral antibiotic but is now primarily used in biological research to study protein transport.