Three adult patients in a single family showed severe myoclonus, ataxia, and pyramidal signs. Enzymatic analysis of lymphocytes, plasma, and cultured skin fibroblasts showed marked deficiency of β-galactosidase activity, more profound with GM1 ganglioside than with another natural substrate, asialofetuin. Other lysosomal hydrolases were normal. Although the physical signs were similar to those of types 1 and 2 GM1 gangliosidosis, none had bony abnormalities.
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