APOE and early-onset autosomal dominant Alzheimer disease: The APOE locus is one of the major risk loci in Alzheimer’s disease, but we found no evidence that Parkinson’s disease individuals carrying one of two APOE ε4 alleles have a significantly higher phenotypic Axis 1 score suggesting that the APOE locus is not a major risk locus influencing Parkinson’s disease clinical presentation (Additional file 1: Fig S17).