Finally, apolipoprotein E (APOE) has been proposed to alter the risk of Parkinson’s disease dementia (Chen et al., 2004; Huang et al., 2006; Goris et al., 2007; Williams-Gray et al., 2009b; Chung et al., 2012; Gomperts et al., 2012, 2013) as well as being a risk factor for Alzheimer’s disease, even if it does not significantly alter the risk of developing Parkinson’s disease without Parkinson’s disease dementia (Peplonska et al., 2013; Multhammer et al., 2014). Here, APOE is linked to early-onset autosomal dominant Alzheimer disease.