Canine degenerative myelopathy, also known as chronic degenerative radiculomyelopathy, is an incurable, progressive disease of the canine spinal cord that is similar in many ways to amyotrophic lateral sclerosis (ALS). Onset is typically after the age of 7 years and it is seen most frequently in the German shepherd dog, Pembroke Welsh corgi, and boxer dog, though the disorder is strongly associated with a gene mutation in SOD1 that has been found in 43 breeds as of 2008, including the wire fox terrier, Chesapeake Bay retriever, Rhodesian ridgeback, and Cardigan Welsh corgi. Progressive weakness and incoordination of the rear limbs are often the first signs seen in affected dogs, with progression over time to complete paralysis. Myelin is an insulating sheath around neurons in the spinal cord. One proposed cause of degenerative myelopathy is that the immune system attacks this sheath, breaking it down. This results in a loss of communication between nerves in lower body of the animal and the brain. Testing
The Orthopedic Foundation for Animals has a DNA saliva test to screen for the mutated gene that has been seen in dogs with degenerative myelopathy. Now that a test is available the disease can be bred out of breeds with a high preponderance. The test is only recommended for predisposed breeds, but can be performed on DNA from any dog on samples collected through swabbing the inside of the animals cheek with a sterile cotton swab or through venipuncture.
The test determines whether the mutated copy of SOD1 is present in the DNA sample submitted. It must be interpreted with caution by a veterinary clinician in combination with the animals clinical signs and other lab test results. The results reported are:

Normal / Normal (N/N, or clear): The dog does not have the mutation and is extremely unlikely to develop degenerative myelopathy. There have been cases, however, in which dogs that tested clear were found to have DM upon necropsy. Normal / Abnormal (N/A or carrier): The dog has one mutated copy of the gene (is heterozygous) and is a carrier but will not have degenerative myelopathy though there has now been some cases of heterozygous carriers developing DM. It will be possible for it to pass the mutation to offspring. A thorough examination of the dogs pedigree and DNA testing should be undertaken prior to breeding a dog with this result. Abnormal / Abnormal (A/A or At Risk): The dog has two copies (is homozygous) for the mutation and is at risk for degenerative myelopathy. Genetics
Breeding risks for degenerative myelopathy can be calculated using the Punnett Square:

If both parents are clear (N/N) then all of the puppies will be clear. If one parent is a carrier (N/A) and one is clear (N/N) each puppy has a 50% chance of being clear and a 50% chance of being a carrier.