First of all, there is a DNA test for the mutant gene called MDR-1. I have mini aussies, and have mine all tested before I even think about breeding them. I would advise doing the test, it is a cheeck swab and $60 but well worth the money.
To put the problem in nut shell, those dogs who have the MDR-1 mutant gene, can no longer block the toxins from several different kinds of drugs from entering their brain. They can have anything from seziures to going crazy similar to a Rabies reaction. I would be happy to give you all the information, and the website where you can obtain a test kit if you contact me privatly. You can email me at firstname.lastname@example.org of use the pm thing on here.
It is very deadly if you dog is mutant/mutant. If normal/mutant they are usually find but I would still stay away from any drugs that could cause a reaction(all drugs on listed on the site I know). A dog tested normal/normal is fine to use anything, and are completely clear of the mutant gene.