In recent years, evidence is emerging showing that carriers of APOE-ε4 alleles have accelerated decline of cognitive function and progression to Parkinson’s disease dementia or dementia in Parkinson’s disease [25–27], and also that there may be an interaction between the ApoE genotype and GBA1 mutations in the development of PD [28, 29]. The gene discussed is GBA1; the disease is dementia.