TREM2 and Parkinson disease: Microglia are associated with a number of neurodegenerative disorders, notably Alzheimer’s Disease (AD)2–4, Parkinson’s disease (PD)5, and motor neuron disease/amyotrophic lateral sclerosis (ALS)6, with many of the genes with highest genetic risk score being expressed in microglia, including; TREM2, CD33, APOE, LRRK2, and C9orf72.