GM1 gangliosidosis is a genetic disease with lysosomal beta-galactosidase 
deficiency caused by mutations of the gene coding for this enzyme. However, 
apparently normal enzyme activity was found in plasma or serum from juvenile GM1 
gangliosidosis patients homozygous for a mutation, R201C (201Arg-->Cys), after 
clotting for 30 min. This extracellular fluid finding is unusual in patients 
with primary and genetic deficiency of beta-galactosidase. The serum enzyme 
activity became relatively low only after 3 1/2-hour clotting because its 
increase in normal controls was not observed in these patients. 
beta-Galactosidase assay is not always reliable, particularly with serum or 
plasma as an enzyme source, for the diagnosis of hereditary beta-galactosidase 
deficiency, unless it is conducted under well-controlled conditions.