DM is an incompletely penetrant autosomal recessive disease, usually affecting dogs 5 years of age or older. The mutation tested for in most breeds is the SOD1:c.118A, referred to as SOD1-A or exon 2. In the Bernese Mountain Dog breed two direct DNA tests are available; one for the SOD1:c.118A mutation and one for the SOD1:c.52T, referred to as SOD1-B or exon 1. The test(s) identify animals that are 'clear' of the mutation, 'at risk' of developing clinical signs of DM and 'carrier' animals, who have one copy of the mutated gene. Dogs shown as 'at risk' or 'carrier' will not necessarily develop the disease. Other genetic and/or environmental factors may influence whether a dog will develop the disease. The test(s) are not diagnostic in nature; DM is a diagnosis of exclusion, which means that other diseases with similar clinical signs have to be excluded. A definitive diagnosis of DM can be only obtained by post-mortem examination of the spinal cord. Owners are strongly advised to ensure that they choose the correct test(s) for their breed as more than one test/mutation may be required for more complete information. Mutations appear to be breed-specific. Current research indicates that the c.52a>T mutation applies the BMD breed only. When the test for SOD1-A is done, a clear result only means that the two genes are clear of that specific SOD1-A mutation; the test for the SOD1-B mutation is necessary to complete the genetic picture. In BMD it has been determined that the SOD1 gene will have either the A or B mutation, not both. If a BMD is at risk for SOD1-A (both copies of the SOD1 gene have the A mutation), then the DNA test for the SOD1-B mutation does not need to be done, and vice versa. If a BMD is clear or a carrier for one of the mutations, then the test for the other mutation must also be done in order to learn the actual status for both copies of the SOD-1 gene.