Conversely, it could be speculated that mutations leading to premature truncation of the protein in the homozygous or compound heterozygous state, in addition to the p.Y888C mutation, may lead to severe progressive neurodegeneration, whereas homozygous missense variants or compound heterozygous variants with a missense and a premature stop variant in the SYNJ1 gene lead to milder phenotypes associated with parkinsonism and a higher susceptibility to seizures. Here, SYNJ1 is linked to Parkinson disease.