GM1-gangliosidosis and Morquio B disease are rare lysosomal storage disorders caused by deficiency of the ?-galactosidase enzyme. GM1-gangliosidosis is a neurodegenerative condition with a phenotypic spectrum that has been classified into three main clinical forms based on onset age and severity. The type I (infantile) form is the most common and severe form with rapidly progressive central nervous system involvement and hypotonia by 6 months of age, visceromegaly, cherry-red spot, white matter abnormalities, coarse facial features and dysostosis multiplex. Type I patients have also been reported with macrocephaly or, less frequently, microcephaly and cardiomyopathy has also been reported in some type I patients. Death usually occurs within the first few years. Type II (late infantile/juvenile) patients usually present between 7 months and 3 years of age with slowly progressive neurological signs including psychomotor delay, hypotonia, locomotor problems, strabismus, muscle weakness, seizures, lethargy and white matter abnormalities. Other characteristic features are dysostosis multiplex, terminal bronchopneumonia and dysmorphic features and skeletal changes that are usually less severe than in type I patients. Type III (adult) patients have the mildest form with onset between 3 and 30 years with cerebellar dysfunction, dystonia, slurred speech, short stature and mild vertebral deformities. Patients with features that overlap the different types of GM1-gangliosidosis have been described. Morquio B disease is a mucopolysaccharidosis characterized by skeletal changes, corneal clouding and impaired cardiac function but no primary nervous system involvement. Patients with phenotypes that are intermediate between GM1-gangliosidosis and Morquio B have also been reported. The incidence of GM1-gangliosidosis is approximately 1 in 100,000 to 1 in 200,000 live births, while the incidence of Morquio B varies greatly from 1 in 75,000 births in Northern Ireland to 1 in 640,000 in Western Australia.