BTK and Alzheimer disease: Human genetic studies over the last decade have identified common and rare variants in genes highly expressed in microglia that contribute to AD risk such as TREM2, CR1, SORL1, CD33 as well as PLCG2, whose protein product phospholipase gamma 2 (PLCγ2) is a BTK substrate (Watanabe et al. 2001; Bertram et al. 2008; Reitz et al. 2011; Sims et al. 2017).