Sensitivity to some drugs is often seen in herding breeds. It is caused by a mutation of MDR1, the Multidrug Resistance Gene, which produces P-glycoprotein protein. This protein works like a pump to remove toxic substances from the body to prevent the harmful effects of toxins. Dogs with this mutated gene may be sensitive to anti-parasitic medications such as Ivermectin, and other drugs, like chemotherapy drugs. Symptoms range from tremors, depression, seizures, incoordination, hypersalivation, coma, and even death. There is no known treatment, but a new genetic test can identify dogs with this nonfunctioning gene. All Australian Shepherds should be screened.