A recent report from the University of Missouri suggests that degenerative myelopathy (DM) in certain breeds of dogs is due to a genetic mutation that is identical to that of humans with Lou Gehrig's disease (amyotrophic lateral sclerosis, or ALS). You can read about this discovery here: http://health.usnews.com/articles/health/healthday/2009/01/22/dog-ailment-als-link-seen.html
For the past decade or more, Dr. Clemmons has been working on a different theory - that DM is a form of multiple sclerosis. In fact, he is certain enough to say "In fact, based upon new data concerning the pathology of MS, we can now say with some degree of certainty that DM is MS in dogs. " (Read more about his work and integrative approach to the diseaes here: http://neuro.vetmed.ufl.edu/neuro/DM_Web/DMofGS.htm).
The good thing about this difference of opinion is that there may be more research money devoted to the problem since these dogs now serve as models for potentially TWO devastating human diseases.
BTW for those who have or work with these patients, there is a test for DM. This is useful because there are other nonpainful paralytic diseases in dogs. For more information on the test and interpreting it, here is the webpage: http://neuro.vetmed.ufl.edu/dm_flash_test_web/index.html