TREM2 and Parkinson disease: TREM2 function can be compromised as a result of rare nonsynonymous variants that cause Nasu-Hakola disease when both alleles are affected (Numasawa et al., 2011, Ridha et al., 2004, Soragna, 2003) or significantly increase the risk of developing AD (Guerreiro et al., 2013b, Jonsson et al., 2013, Sims et al., 2017), behavioral variant frontotemporal dementia (Guerreiro et al., 2013a, Le Ber et al., 2013), semantic variant of primary progressive aphasia or occasionally Parkinson's disease (PD) (Borroni et al., 2014, Liu et al., 2016, Rayaprolu et al., 2013) when 1 allele is affected.