This report gives a more complete description of pathologic and enzymologic findings in the feline Maroteaux-Lamy Syndrome based on additional studies in the cat originally described, three other affected Siamese cats from two additional families and an affected kitten produced by an experimental mating between obligate heterozygotes from two independently ascertained families.All affected animals had facial dysmorphia with a small head and broad, shortened maxilla. None had hepatosplenomegaly. All had a diffuse ground glass appearance of all layers of the cornea.All long bones had multiple severe exostoses with epiphyseal dysplasia and irregular articular surfaces.There was no indication in these cats of any deficit comparable to what is defined in man as mental retardation.The electrophoretic patterns obtained showed that the primary glycosaminoglycan (GAG) excreted in the urine by the affected cats was dermatan sulfate.In two human mucopolysaccharidoses (MPS) VI patients and all five affected cats, 90–100% of the circulating polymorphonu-clear (PMN) leukocytes contained, by light microscopy, excessive coarse granulations in the cytoplasm. In the human MPS VI patients, most PMN leukocytes contained, by electron microscopy, small (0.2–0.5u) round membrane bound inclusions, a majority of which contained an amorphous to granular electron dense material. The predominant inclusion type in the cat varied among families: lamellar, granular, and crystalloid, but all three inclusion types were present in some cells of all affected individuals.The data indicate that the detection of normals, heterozygotes, and homozygotes for the mutant gene was possible using an assay for arylsulfatase B (ASB) in peripheral blood leukocytes. The existence of an obligate heterozygote which has very low ASB activity with the nitrocatechol sulfate assay indicates that clinical criteria must be combined with assays of enzyme activity before an individual can be defined as affected.Pedigree information in the three independently ascertained families of cats is consistent with autosomal recessive inheritance. An experimental mating between obligate heterozygotes in families 2 and 3 produced a kitten with characteristic signs and enzymatic deficiency of the syndrome, indicating that the same locus, and in all probability the same mutant allele, is involved in these two families.SpeculationThis feline model of mucopolysaccharidosis VI (Maroteaux-Lamy Syndrome) can be produced by experimental mating and should allow advances in the understanding of the pathogenesis and approaches to therapy for this and related genetic storage diseases.