In 2009 a DNA mutation was found that explained the observed DM cases (). A cheek swab DNA test is available for this simple recessive mutation - meaning that a dog has to have two copies of the DM-allele - one from each parent - to be consider DM-affected (at high-risk for developing DM).
A March 2014 paper in the Journal of Veterinary Internal Medicine () has reported that this DM-allele has been found in 124 breeds so far.
1. One of the breeds where the DM-allele was found is Norwich terriers. The paper reports that 74 Norwich terriers were tested and 16 were found to be DM-carriers (no DM-affecteds).
2. NTCA member Denise Faulkner has made the DM-carrier status of a Norwich that she owns () public on the OFA web site (www.offa.org).
3. The Norwich Breed Club Health Survey on the OFA web site () lists one Norwich terrier with DM - although we do not know who the dog is or how the diagnosis was made.
I conclude from this that the DM allele is in the Norwich gene pool, and that I can use the DM DNA test to reduce the chances that I will breed a DM Norwich. All I have to do is never breed a DM-carrier to a DM-carrier. Of course, the only way to know the status of a dog is by testing.
In my opinion, DM DNA testing joins PLL (primary lens luxation) DNA testing as something done by responsible Norwich breeders.
 Genome-wide association analysis reveals a SOD1 mutation in canine degenerative myelopathy that resembles amyotrophic lateral sclerosis, Awano et.al., Proceedings of the National Academy of Sciences of the United States of America, Feb 2009.
 Breed Distribution of SOD1 Alleles Previously Associated with Canine Degenerative Myelopathy, Zeng et.al., Journal of Veterinary Internal Medicine, March 2014.
 Denwich The Lion Hearted (Leo), RN16047102.