Small-molecular nonenzymatic glycoproteins are necessary for degradation of sphingolipids in lysosomes. GM2 activator encoded by a gene on chromosome 5 is essential for hydrolysis of ganglioside GM2 and its asialo derivative. Mutations of this gene cause Tay-Sachs disease-like clinical phenotype (GM2-gangliosidosis AB variant). Another gene on chromosome 10 codes for a protein called prosaposin, which is then processed to four related but independent activator proteins; saposin A, B, C, and D. These five proteins show different molecular functions, and their mutations cause diseases with prosaposin deficiency (clinically similar to Gaucher disease type 2), and saposin C deficiency (clinically similar to Gaucher disease type 3). They are rare genetic disorders, but provide information about molecular events between the enzyme and substrate in lysosomes.