Hexosaminidase deficiency diseases or GM2-gangliosidoses were originally described as infantile encephalopathies. Recently, hexosaminidase deficiencies have been found with different phenotypes, including juvenile and adult encephalopathies, cerebellar ataxias, and motor neuron diseases. Individual cases have resembled Ramsey-Hunt syndrome, olivopontocerebellar ataxia, Friedreich ataxia, amyotrophic lateral sclerosis, Kugelberg-Welander disease, Fazio-Londe disease, and Charcot-Marie-Tooth disease. Tremor, dystonia, spastic paresis, and psychosis have been seen. Since few diagnosable causes for these system atrophies are known, these patients should be tested for hexosaminidase deficiency. These recessive disorders fit a multiple loci/multiple alleles genetic scheme, and a clinical genetic classification is presented.