Table 1 provides a list of most of the studies that have been published in the PUBMED database with the key words of “microglia,” “phenotype,” “PET,” and “Alzheimer’s disease.” The 18 kDa translocator protein (TSPO) is the main target for PET studies, and is mainly expressed by activated microglia (Cosenza-Nashat et al., 2009). The gene discussed is TSPO; the disease is Alzheimer disease.