MDR1 Gene Mutation:
The MDR1 gene encodes P-glycoprotein, a drug transport
pump that plays an important role in limiting drug absorption and distribution
(particularly to the brain), as well as enhancing the excretion of many drugs used in dogs.
Some dogs, particularly herding breeds, have a mutation in the MDR1 gene, leaving them
defective in their ability to limit drug absorption and distribution. These dogs also have
delayed excretion of drugs that are normally transported by P-glycoprotein.
Drugs Affected by the MDR1 Gene Mutation:
Acepromazine Butorphanol Doxorubicin Erythromycin Ivermectin Loperamide Milbemycin
Moxidectin Rifampin Selamectin Vinblastine Vincristine
Breeds affected by the MDR1 mutation (frequency %)
Approximate Frequency
Australian Shepherd 50%
Australian Shepherd, Mini 50%
Border Collie < 5%
Collie 70 %
English Shepherd 15 %
German Shepherd 10 %
Herding Breed Cross 10 %
Long-haired Whippet 65 %
McNab 30 %
Mixed Breed 5 %
Old English Sheepdog 5 %
Shetland Sheepdog 15 %
Silken Windhound 30 %
Testing a Dog for the MDR1 Mutation is Easy:
Order a testing kit by contacting the Veterinary Clinical Pharmacology Laboratory at
Washington State University:
or by phone 509-335-3745