It has become increasingly clear that "simple" recessive disorders provide unique insight into the complexities of common diseases. For years, research on Gaucher disease, a rare inherited disorder resulting from a deficiency of the lysosomal enzyme glucocerebrosidase, focused on its cell pathology and genetic basis. Clinical research showed that Gaucher disease manifests with broad phenotypic variation typical of many metabolic disorders, ranging from neonatal lethality to asymptomatic octogenarians. This clinical spectrum now overlaps with different disorders including Parkinson's disease and other Lewy body disorders, myoclonic epilepsy, and infantile neurodegenerative disorders. In fact, unraveling the factors contributing to heterogeneity in a single gene disorder may have a direct impact on studies of the pathophysiology and therapeutic options available for these more common and complex neurologic diseases.