Hypomyelination, commonly known as shaking puppy syndrome, effects the myelination of the nerves of the central nervous system. Symptoms can vary by breed, but primarily include a loss of precise muscle control manifesting as tremour, especially in the hind legs. In puppies, this can cause problems with feeding. Symptoms in puppies may worsen with excitement, and decrease when puppies are less active. In some breeds, this is reported to be progressive to fatality. In other breeds, this is reported as diminishing in severity with age. Age of onset, present from birth with puppies appearing normal until 1-2 weeks of age on average. Exact age of onset can vary. Between different dog breeds, inheritance may vary - it is suggested in Springer Spaniels that the disorder has an X-linked inheritance, in other breeds the disorder is autosomal recessive.