Interestingly, however, the load of MAO-B enzyme in the isocortex, as imaged with PET, appears to vary between and within individuals at different stages of AD, possibly as a result of reactive astrocytes in the human brain [27, 31], which adds to the complexity of in vivo imaging with the developed tracers, especially for the first-generation tracers. This evidence concerns the gene MAOB and Alzheimer disease.