BACKGROUND AND INTRODUCTION Lysosomal storage diseases (LSDs) affect normal lysosomal catabolism, and are classified as inborn errors of metabolism 1. In nearly all cases disease results from genetic deficiency of lysosomal hydrolases. These hydrolases are responsible for the stepwise catabolism of macromolecules targeted to the lysosome for degradation. Each enzyme in the process depends on the action of the previous enzyme to alter substrate so that it can be recognized by the next enzyme in the pathway. Failure of one enzyme leads to lysosomal accumulation of incompletely digested substrate. These diseases, of which over 40 have been described in humans, have been recognized to occur in veterinary patients since the early 1960's 2. The attention that LSDs have received in veterinary medicine is due to both the striking pathology, and to their importance as animal models of human disease 3. Lysosomal storage diseases have been identified in virtually every major veterinary species with the exception of the horse 4. All forms are inherited as simple autosomal conditions except two X-linked diseases1. To date at least 21 different forms of LSDs from over two dozen different breeds have been identified in dogs and cats (reviewed in 4-7). The molecular genetic defect has been defined in over 20 such cases8-25. In at least three breeds, more than one form of LSD have been documented. These included the Korat (GM1 and GM2 gangliosidoses)9,26 and Siamese (GM1 gangliosidosis, and mild and severe forms of MPS VI)27,28 cats, and English springer spaniels (GM1 gangliosidosis and -fucosidosis)29,30. In at least one condition, Krabbe disease, two closely related breeds have the same mutation, Cairn and West Highland white terriers12.