Certain chemical dyes injected into the peripheral circulation infiltrated most organs with one an exception, the brain.  This led to the well know concept of a blood-brain barrier. Janet Ryland of Animal Genetics describes a deletion in multi drug resistance gene, most important drug efflux system of the barrier, to be responsible for dogs’ sensitivity to a wide range of medications including anti-parasites, cancer drugs, sedatives, anti-diarrhoea, pain medications and heart drugs.