TREM2 and Parkinson disease: There are already several well-established associations between neurodegenerative disorders and heterozygosity for several genes of varying function, including APOE4 (6), TREM2, ABCA7 and SORL1 (7) and Alzheimer’s disease (AD), amyotrophic lateral sclerosis (ALS)/frontotemporal dementia (8), and LRRK2 and Parkinson’s disease (PD) (9).