TREM2 and Parkinson disease: In addition to Alzheimer's disease, R47H and other variants of TREM2 are reported to be risk factors for PD (Benitez et al., 2013; Rayaprolu et al., 2013; Liu et al., 2016), although this remains controversial (Jonsson and Stefansson, 2013; Lill et al., 2015).