GM1 gangliosidosis and Morquio B disease are distinct disorders both clinically 
and biochemically yet they arise from the same beta-galactosidase enzyme 
deficiency. On the other hand, galactosialidosis and sialidosis share common 
clinical and biochemical features, yet they arise from two separate enzyme 
deficiencies, namely, protective protein/cathepsin A and neuraminidase, 
respectively. However distinct, in practice these disorders overlap both 
clinically and biochemically so that easy discrimination between them is 
sometimes difficult. The principle reason for this may be found in the fact that 
these three enzymes form a unique complex in lysosomes that is required for 
their stability and posttranslational processing. In this review, I focus mainly 
on the primary and secondary beta-galactosidase deficiency states and offer some 
hypotheses to account for differences between GM1 gangliosidosis and Morquio B 
disease.