Brefeldin A is a lactone antibiotic found in fungi such as Eupenicillium; it exhibits anticancer and antiviral activities. Brefeldin A inhibits activity of guanine nucleotide exchange factors, preventing association with COPI and COPI-mediated vesicle formation and suppressing protein transport from the endoplasmic reticulum to the Golgi apparatus. Brefeldin A is used in research models to study endoplasmic reticulum stress and protein transport. In vitro, this compound inhibits poliovirus replication. In prostate cancer cells, brefeldin A inhibits cell growth, induces apoptosis and G1 phase cell cycle arrest, decreases levels of p53, CDK2, CDK4, and cyclin D1, and causes cell death.