SNCA and Parkinson disease: The theory that age-dependent GM1 ganglioside deficiency is one trigger for idiopathic PD is attractive not only because it is supported by clinical and animal data but also because it recognizes that there are many potential mechanisms besides GM1 ganglioside loss that could drive trophic signaling failure that in turn would lead to accumulation of α-synuclein in neuronal membranes and activation of innate immunity.