TREM2 and Parkinson disease: Numerous genes associated with Alzheimer's disease (AD), Parkinson's disease (PD), motor neuron disease/amyotrophic lateral sclerosis (MND/ALS), and frontotemporal dementia (FTD) are expressed in microglia, including TREM2, CD33, LRRK2, and C9orf72 (O'Rourke et al., 2016, Russo et al., 2014, Villegas-Llerena et al., 2016), prompting a growing interest in microglia biology and their relevance to neurodegenerative disease.