CD33 and Alzheimer disease: In a study that involved both CD33 knockout mice and human brain samples, the density of CD33-immunoreactive microglia positively correlated with Aβ burden and Alzheimer’s disease patients had a 5-fold increase in CD33 mRNA relative to controls.10 Another study reported that the risk allele is associated with a 7-fold increase in CD33 cell surface expression of circulating monocytes.44 The strong negative correlation between CD33 expression and FDG PET SUVR observed here (Fig. 3) echoed the finding that elevated CD33 expression may increase risk for Alzheimer’s disease.