For example, APOE-e4 may play a role in nitric oxide regulation in the brain (Vitek et al., 2009), while mutations or single-nucleotide polymorphisms in innate immune genes such as TREM2 (triggering receptor expressed on myeloid cells 2) (Liddelow et al., 2017), CR1 (complement receptor 1) (Efthymiou and Goate, 2017), and CD33 (Karch and Goate, 2015) have been associated with altered microglial function in the context of AD. Here, APOE is linked to Alzheimer disease.